Frankly, I get a little tired of the whole 'nature vs nurture' twin trope that gets recycled so often. It's like every author thinks twins exist solely to debate genetic destiny. That said, 'The Girls' by Lori Lansens handles identity in a way that stuck with me. It's a fictional memoir written by conjoined twins, Rose and Ruby. Their physical bond forces a shared life, but the narrative voice—they take turns writing chapters—slowly reveals how wildly different their inner worlds are. Rose is pragmatic, a bit resigned; Ruby is more whimsical, observing details Rose misses. Their personalities aren't opposed for dramatic effect, they just naturally diverge because they're two separate people who happen to share a body. The book isn't about one stealing the other's life or some secret swap; it's quieter, about how identity forms even under the most constrained circumstances.
Another one that comes to mind is 'Her Fearful Symmetry' by Audrey Niffenegger. Okay, yes, it has a ghost and a seance, so it's a bit gothic, but the core is these American twins, Julia and Valentina, inheriting a flat next to a London cemetery. They've built their identities in opposition to each other—one dominant, one passive—and moving to a new country without their parents forces a painful, literal uncoupling. The personality differences start as a comfortable, codependent system and become a cage. The supernatural element just heightens the tension of whether they can ever truly become individuals. It’s messy and the ending is divisive, but it captures that suffocating feeling of being seen only as half of a set.