Interestingly enough, I have direct experience of an example of this phenomenon.
About 3 1/2 years ago, in October of 2006, a close friend of mine came over to my college campus where i was living at the time and expressed a desire to take a psychedelic. I had recently obtained some 2C-E, which neither of us had ever tried before, and he decided to take some, but I decided not to trip with him because I had an exam the next day. We did some Erowid/PIKHaL research and he decided to take one of the 20mg capsules that had been sold to me (I have since realized that 15mg is a much more reasonable dose of 2C-E). What followed was totally bizarre and to this day probably the most extraordinarily transformational trip I've ever witnessed in another person.
First, some background: my friend was a freshman in college at the time, and had moved to the city where I lived a couple months previous to start school. The previous summer, he had made a series of extremely unpleasant gestures towards many of the friends he had in his hometown (many of whom were friends of mine as well), and he had been almost entirely rejected and ostracized by most of his community of friends because of how much of a dick he was acting. Everyone had noted his general descent into really unpleasant, selfish, careless asshole behavior over the previous year or so, and the summer before he went off to college, it came to a head and he was basically run out on a rail. When he moved to my hometown for college, I was basically his only friend. His recent experiences in his hometown had been weighing pretty heavily on our conversations whenever we hung out and it was clearly still an issue that neither of us really felt closure on. I had also recently introduced him to psychedelics and he had become EXTREMELY enamored of them, and had tripped on mushrooms, acid, and 2C-B a total of probably 4 or 5 times in the two months leading up to this event. He had just tripped on 2C-B for the first time the previous weekend.
So back to the story: my friend took some 2C-E and the two of us walked around my college campus and found a quiet spot to sit and burn a joint. While we were smoking, he noted that he was coming up, but complained that he was not getting any visuals, even though he'd heard that 2C-E was a visual drug. I mentioned that it might help if he closed his eyes. He did, and then immediately jumped up and screamed. I asked him what had happened and he explained that the previous weekend, while he was tripping on 2C-B, he had looked in the mirror and found himself compelled to make a certain evil grinning face, and that he had made this face involuntarily every time he'd looked in a mirror for the entire week since, and that when he had just closed his eyes, he'd seen this face looking at him and talking to him.
This alarmed me, understandably, so I started asking him about it and parsing through his experiences, and over the next couple hours he developed an entirely separate personality that I could talk to independently of the person I was used to talking to when I hung out with him. Furthermore, when I spoke to this separate "evil twin" of his, he had a different voice, different body language, and even different memories, and I could VERY easily distinguish between the two ... it was like night and day. There was no mistaking which one I was talking to at any given time. They talked to each other in front of me as well. The "good" one explained to me that because I was trying so hard to help him emotionally, the "bad" one (which had apparently been guiding him through the recent period of him acting really shitty and unpleasant towards everyone) hated me, and even wanted to kill me. This got to the point where he literally put his hands around my neck, and looked at me with pure hatred in his eyes and said that as much as he wanted to, he couldn't bring himself to actually strangle me (the only thing I could think of to do was sit there and express trust in him, in the hope that he'd be overcome by the good will I was showing and the "bad" side would break down. It seemed to work).
He explained that during the time that the "bad" side of him had been guiding his decisions (a period of several years, apparently ... and the "bad" side of him had apparently existed ever since the trauma of losing his father to cancer at age 10), he would spontaneously black out every so often and wake up with a poem in front of him that he did not remember writing. Since his poetry seemed to be the way that the "good" side of him expressed itself while the "bad" side was "in charge," so to speak, I decided that the best way to deal with the situation was to take my friend to a relatively private place where there was a mirror, and give him a book of poetry ("The Prophet" by Khalil Gibran, an absolute masterpiece) and force him to read it to himself in the mirror. He read the entire book to himself over the course of his comedown, despite intermittent arguments and wrestling matches with his "bad" self, and he came out of the trip having successfully exorcised himself. He woke up the next day and called me to tell me that it was the first morning in years that he had woken up without voices in his head. To this day I believe that he has been far more sane and balanced since that trip than he was beforehand.
I don't know what to think with regards to how "real" the experience was ... maybe the "alternate personality" was just something that happened because he was tripping, and maybe it really had "been there all along" ... honestly, it doesn't matter to me. It was real as shit no matter what. All I know is, my friend split in two that night, and kicked out the half of himself that he didn't want around anymore. It was harrowing but it was extremely healing for him.