Funny that this thread should come up now, as I'm at the end of a game I've been playing on my more recent survival horror binge, which actually featured the main character's personality swapping as a gameplay feature... and failed miserably at it.
The game in question is Clock Tower II: The Struggle Within. The game is centered around a girl named Alyssa, who has an alternate personality named Bates. However, while Alyssa is a schoolgirl with typical 90's horror game voice acting, Bates is a flaming, raspy, male voiced serial killer. No, he's literally on fire. All the time.
So the idea is that when Alyssa is faced with something stressful (enviornmental hazards, enemies, etc.), if she's not wearing her father's amulet (which can be set down virtually anywhere), then she will be taken over by Bates, and the player continues as him until the amulet is retrieved. Aside from the obvious differences, each plays a little different in encounters. While Alyssa must hide from her aggressors, Bates just shoots people.
Here's the problem; certain sections of the game can only respectively be completed as Bates or Alyssa. It's impossible to determine when you need to switch aside from walkthroughs or sheer luck. The same guy that instantly kills you as Alyssa will recieve a brutal kick in the testicles from Bates. Useful information can only be extracted from a suicidal nurse as Bates, rather than Alyssa for some reason. The fact that it's not up to the player who they spend the most time as is a missed opportunity.
The game's enjoyable, more in a B-Movie cheesy sense than anything else, but they had a bunch of cool things that they could have done with the multiple personalities mechanic that they missed out on.
Although, I will mention that Bates' voice acting is surprisingly good. His lines are the cheesiest in the entire game, but the voice actor manages to mask this very well.