I'm a fan of the idea that a large part of personality is rooted in physiology (primarily brain structure, neurochemistry, and hormones), so the idea that a single brain with a single set of life experiences can house two or more radically different personalities is quite bizarre to me. If it was more widespread and not so easy to fake it would make for wonderful psych studies.

And of course, even the concept of personality is pretty subjective. There's some theories that say all people have multiple similar but not the same personalities for different contexts. It would be very hard to tell.