Ok this is gonna sound really weird but my thought on this is that David is the one with the problems. And the problem may not even be MPD as all of you put it. If i'm right it looks like he has schizophrenia more then anything. I say this because, lets face it, Oliver has a mild mental lapse. He doesnt under stand what is going on most of the time and because of that in school people blamed him for acting crazy in class and acusing him that he just blacked out and never remembered it. Which could have been easily pulled off since OLIVER AND DAVID ARE TWINS. And most of the time mental problems are genetics so I figured that David got it from his father since it seems that their father is living a double life.
Also in this paragraph either Dom made a mistake or it clearly shows that David and Oliver are 2 different people:
Holding her hand over her fast-swelling black eye, she moved cautiously back to her bedroom to think for a moment. Damn Brian! She had been completely unprepared to be awakened in the middle of the night as David snuck back through his bedroom window. She'd been afraid that oliver had had a bad dream and went to check on him, but ended up with a confrontadtion to deal with instead and she