I believe you're referring to Dissociative identity disorder (DID), or multiple personality disorder (MPD), not Schizophrenia, and a friend who's a psychiatrist told me that in quite a few decades of practicing, he hasn't encountered even once such a diagnosis so he doubts it even exists outside of Hollywood movies.
As far as I'm concerned what matters is how many entities exist who display a behavior of a separate entity (at least to the degree that we acknowledge as being separate) - i.e display different personalities each with a cohesive structure and integrity. It really doesn't matter how many physical brains they share.