There's a short science book called Hidden Guests, it talks about why women have the potential to end up with microchimeric "incursions" from sexual partners and their own fetuses, fetuses can have microchimerism with each other (not just twins but prior fetuses from same mother) and fetal cells crossing into the mother and towards the end talks about the resemblance between this and transplants( someone else's cells thriving in one's body). So if organ transplants can potentially have that effect, it's already happening to sexually active women, with the caveat that for the fetal cells it wouldn't have much formed personality, yet.