Schizophrenia and Split Personality
If we see consciousness as a software phenomenon we can see it is not hard to realize that brains as our biological hardware are capable of running more than one consciousness.
This is similar to the Bicameral Mind Hypothesis.
It makes perfect sense.
Why does it happen? If you put a system under extreme stress then systems try and come back into equilibrium. When things are impossible to reconcile our brains are capable of being contorted into very very odd patterns.
In essence this is ‘mental illness’. See https://interfaceware.atlassian.net/wiki/x/pQCSpQ.