With that out of the way, lets look at the person who first mentioned knowing of someone with more than one talent. While they're transformation into talented people and using their talents gives her access to all sorts of talent effects, they're not really "her" talents. Rin is capable or shapeshifting into animals and other talented individuals, but not talentless humans^[1]. This is stated very clearly multiple times. But why can't she transform into talentless humans?

"It would make her even more OP"

Is it for balance reasons,? Why give one of the most powerful talented people alive already another ability? What would being able to transform into a talentless person even do for her?

• She could go undercover and take out a politician? There are plenty of others that will take their place after and use the assassination as an excuse to strengthen the oppression.
• Influence a politician? Rin's acting skills are decent enough to trick a sleep deprived Nana into thinking she was Michiru and falling asleep near her, but the second she started acting unlike the Michiru, Nana was aware of the possibility she was a shapeshifter. Not only would a politician not believe that their advisor who has been saying one thing this entire time suddenly says the opposite, but even if they do fall for it their government has political parties and it's not like the rest of the party is just going to go along with it instead of using the change as an excuse to oust that politician from the party and take their spot for themself.
• Become the politician? Oh and have a bodyguard staring at her 24/7 locking her in that form until they realize that this person is not acting quite like the same person they get paid to watch 24/7 usually does and suspects something, which she is now left as a sitting duck without access to any talent and no way of escape? Rin is not powerful because she can shapeshift, she's powerful because she can use talents.
  If anything, having the option to even try to do one of these things that will likely get herself killed would be a nerf if she ever gets overconfident and try one.

"Talented People aren't human"

The thematic reason that people are instantly going to assume is that "Talented people aren't human" but not only is that not the direction the story seems to be going, but we can actually determine whether or not they are. The modern human is considered to be and member of the species "Homo Sapien." What "Homo Sapien" means is irrelevant^[2], what we're actually interested in the word I put right before it, species. Words have definitions. And they are definitions that must abide by the rules of our world because it is something that comes from our world. So long as a story is written in any language from this world, it must follow that definitions that language provides. It can specify that in this world, it means something else, which gives the word an additional meaning, such as the word "talented" meaning a person who has special powers, but the concept itself is still new and there is a very clear distinction between someone who is "talented" in the way the series provides, and someone who is talented by our definition. The word "Human" is still being used to refer to the same concept as we use the word for even in the Talentless Nana world, so we can apply our definition to it and answer the question, "Are talented people human or not?"

What is the definition of species though? Well at least in English, a species is defined as "The largest population of a reproducing group possible where any two members with proper sexual function and of the appropriate sex may produce a fertile offspring."^[3] If you can take one human who is capable of having children, one member of an unknown species of the appropriate sex who is also able to have children, and the two of them produce a child who is also able to reproduce, then that unknown species is also human. To put it in simpler terms, so long as a single person meets any of these sets of criteria, talented and talentless are the same scientific species:

1. Someone with one talented, one talentless parent, and any number of children of their own
2. A talented individual with two talentless parents as well as any number of children of their own
3. A talentless person with two talented parents as well s any number of children of their own
   While the requirement to be able to have children themselves is something difficult to prove due to the majority of the cast being of the same generation, and Nana may be category 3, we can't say for sure if she is fertile. However, as the talented people we have seen thus far have only existed in universe for approximately a single century^[4], whoever that first talented person was had two talentless parents, and as we know talented people have had children of their own, one of these earliest talented people would have to be category 2, fulfilling the criteria to prove that Talented and Talentless people are the same species, and thus talented people are canonically human.

The Real Reason + The Mechanics of Shapeshifting

So, what's the real reason she can't turn into a talentless human? Well, in order to actually do so, part of your body will need to be moved to a different spot, for example when she turns into a cat, part of that means she needs to move her ear onto the top of her head, as well as change it into the shape of a cat ear. She must do this for every single body part, or more specifically every single cell of her body. Rin's shapeshifting is her being able to rearrange her cells instantly to where ever she so desires. Wouldn't that mean she could transform into a talentless human though? Well, technically yes, she probably could in theory shapeshift into a talentless human with this mechanism, but also not really. She needs to actually know an animal or talented person to shapeshift into them, we know that much, but how many talentless people does Rin know? She would get sick fairly often, you can even see how unhealthy she looks. She even looks like they haven't been feeding her while she was captured. Wait, feeding, what was Rin's diet back on the island again? Stolen coffee and milk from a vending machine. Rin is SMALL. And she was designed this way for a reason. She cannot transform into a talentless human because she doesn't have enough cells in her body to transform into any talentless human she knows. This also explains why she can't turn into plants or inanimate objects, because plants have very different cells than animals and inanimate objects don't have any, as well as her always transforming into smaller animals, like cats, birds, and butterflies, but never larger ones like and elephant or lion. A trip to the zoo would be a no brainer to be able to walk into a cramped vehicle and turn into an elephant to destroy it instantly. That could be even stronger than turning into certain talented is. And why does she need to know the animal at all before transforming into them? Because if she doesn't have the reference image to go off when putting herself into that form she could very well get something very wrong and turn into a creature unable to perform the functions necessary for life.

She must be able to somehow do something with any excess cells, hence why she can go smaller but not bigger, whether that's because she can somehow overlap them and keep them inside somehow or she can store them elsewhere, either of which makes sense for someone who can choose where every single cell of her body ends up, though we don't see and lumps of flesh anywhere when she turns into an animal, and being able to store them further away would allow for teleportation just with her own talent, so she must be able to either combine or them squish together. This might even explain why someone who apparently is a fantastic chef chooses to have such a terrible diet, it's just not comfortable for her to squeeze herself in so much so she just makes sure she has less of herself to squeeze at all.

Rin's Second Talent, "Referencing"

Why is she able to transform into Jin Tachibana and any other talented person that she knows? They are all larger than her so what's the deal? Remember what we said earlier? If a person is able to do something that should not be possible, even with whatever low order ability they may get from their talent, then it's a different talent, even if it's similar in effect to the first. Rin being a dual talent user also explains why she said that she's unaware of anyone with more than two talents when revealing that she wasn't actually Jin Tachibana, because if her shapeshifting really all was a single talent then she very would could have been a shapeshifter and a telepath if two talents is the maximum.

Mechanical Inferencing

The mechanism as to why Rin can transform into other talented people isn't something I know for sure, but it seems to be something to do with some sort of connection that forms between them. Maybe we have all the information needed to figure that out already, or maybe there's a piece left to be found. I do have an idea about this, but it's not one I can give any evidence for currently. I suspect she is somehow able to act as some kind of "Reference Variable"^[5] for the person she changes into.

So to break it down, this is me theory about how her second talent works:

1. She is able to "target" any talented person whom she knows.
2. She is then able to create that person's body, complete with the appearance, physical ability, and talent, overriding her own.
3. She cannot access any sort of cognitive function, so her personality, knowledge, memory, and thoughts remain as is, her own and not the targets.
4. The targets current pose, position in space, and state on consciousness are all uncopied.
5. Should she change her target (which can only be done while unseen for an unknown reason), this will be reflected similarly to the previous target
6. If something happens to her target that changes their appearance, physical ability, or talent, this is immediately in effect in Rin as well.
7. As she isn't actually turning her body into the target but rather to a reference of it, she is unable to use their cells to contribute to her original transformation talent, as the amount of cells in her body, which is the one that has that talent, remains the same
8. In order to not transform, she simply targets herself.

We know her talent targets who she transforms into as she states before using it on Nanao Nakajima,^[6] and we know what she does and doesn't gain from her target based on what does and doesn't change. We don't know the exact details with how she changes targets or why she must be unseen, but we can assume that this is a hard limit and not her own choice unlike her not transforming into talentless humans, meaning whenever she is being observed, she is simply unable to use it, period. To continue the code analogy, perhaps someone observing her somehow loading her into memory, either their own or the universes', which prevents her from reassigning what target she is referencing? Is Talentless Nana some sort of simulation? No idea, I'm just using comparisons to things I'm familiar with. What interestingly is not a hard requirement, according to my proposed mechanism, is her inability to turn into the dead, and this is one I actually have some evidence for:

1. As Jin Tachibana was on life support and unable to die from natural causes, and was in a secret location, she knew his safety was true up until Tsuraoka discovered his location
2. Additionally, as Kyoya is immortal, she can also always be aware of his safety at any given point in time. This covers the two most common targets for her transformation.
3. Apart from these two, we only ever see her transform into Michiru, Seiya, Moguo, which she does on the island while observing the only currently active murder that could threaten their safety, Nanao, who she would be unable to transform into if he were dead as his corpse would continue to nullify any talent targeting it, and are told of her transforming into Tsuraoka who was very unlikely to have died without warning given his position in the government
4. What is noteworthy, is how after being broken out of her cell, she only transforms into Jin Tachibana a single time in the first escape attempt, where she is visibly apprehensive before doing so. She takes four whole panels before she does something she has previously done nearly instantly. Should I be correct, this is because she is unsure of Jin Tachibana's safety, as Tsuraoka could have very well killed him anyways, meaning by transforming into him, she would be risking turning into a dead body, and while we only see a handful of info relevant to how death works, she is certainly unaware of the answer to whether or not she is able to change her target while she is a dead body with no cognitive ability. She also has not become Jin at all during the second attempt, as she does not have the memory of the first and is still unwilling to take the risk until it becomes a requirement again.

So there you have it. The only thing I don't have an answer for is why she cannot transform while someone is looking at her. It could very well be as simple as her not wanting to though, or there could be some outside force preventing it, but this gives us a solid explanation of exactly how her talent works, and why it's actually two separate talents with a similar effect, a trend we'll see in not just one, nor two, but three additional characters, with potentially more still to be seen.