I ADORE your interpretation of where you think this is going and I would be excited as hell if it actually turned out this way. I know many fans would be disappointed if we were to find out that it was a case of MPD or schizophrenia but damn it would explain everything beautifully. Even now, coming up toward the end of season 9, this still fits. Dean is getting darker by the day and Sam, even though he is still there for Dean whenever it counts, is pulling away from him. It's like one part of a dual mind that is desperately hanging on and will stop at nothing to achieve it, while the other is breaking away/ready to die off. But if it does turn out that way, which personality will win out? Would we have a final merging where the person finds out it wasn't real, or would one personality win out over the other and 'kill off' the other leaving that person as either Sam OR Dean with a totally cracked psyche locked forever in a loony bin? You have to remember, in the bible, Cain kills Abel so he can go to heaven. Will 'Dean' kill 'Sam' in his ultimate goal to save him?

I love your mind, I want its babies!!!