Schizophrenia is not about more than one being existing in the same body, and multiple beings don't exist within the same mind. Even the name of the illness that you meant is Dissociative Identity Disorder, defined as "a disorder characterized by identity fragmentation rather than a proliferation of separate personalities" (source).
I used to know lot more about this phenomenon schizophrenia in 1970's. It was then described and understood as consisting of sometimes quite opposing and independent aspects in one person, and even independent personalities in one person, or aspects and personalities which manifest periodically in the life of one person. It is a multifaceted phenomenon, its desriptions have changed in the course of centuries. It is understood differently in different cultures, its descriptions and the understanding of its possible causes differ in the many countries of the modern and ancient world. You can't say that any one definition is "correct", or that it really captures its essence.