My first Malkavian character (when play testing the LARP system back in '92), David Emmanuel Moseby, started as a human with Obsessive Insomnia as a "derangement"/neurological problem, who after becoming embraced (by another PC), shattered mentally, and over the course of the following 2 years developed MPD (now labeled DID), largely in response to the world around him and his connection to the Malkavian Madness Network. Each personality (most of whom answered to the same name, but were radically different in terms of personality and personal "history") had a derangement, themselves, and each was in their own way a piece of the larger "puzzle" of reality, though they were only subconciously aware of each other, and acted independantly on a conscious level (much as how people/kindred do, at least from David's POV). It was a similar case where the individual personalities had different advantages/weaknesses (including Negative Traits), but I as a player used what "points" were given me to "build" a character sheet within the same parameters as anyone else (in fact, over the first year, I "assigned" myself Negative traits and Derangements to spice things up; when the ST's found out that I had done that, and hadn't bothered with any additional traits elsewhere, they thought it was absolutely righteous, and told me I could do whatever I wanted as they trusted my judgement). Some personalities had abilities that others didn't, but from a game mechanic perspective, all abilities and powers were paid for normally, as if David were just a regular ol' single-minded character, with the only advantage being that the ST's gave me leave to pick up a dot or two in an out of clan discipline as long as it made sense to the personality in question. The only out of clan disciplines that he developed were Celerity and Presence, which were far from special or unique.
The real key to playing a character like that is to understand that any "game-mechanic" advantages stemming from the vampire's derangement(s) should be justified and paid for normally, with perhaps just a small amount of leeway given to allow for good roleplaying.
An EXCELLENT example of how such "skill advantages" or "ability redistribution" can take place, but still come through the fractured mirror of a "drawback", or "disadvantage", is the Showtime series "United States of Tara", an absolutely wonderful show that not only portrays the more extreme manifestations of MPD/DID (Toni Collette's Best Actress Emmy win was richly deserved), but shows how, in game terms, the "insanity" of a Malkavian can often seem "sane" when compared to the neuroses of "normal" people (a subtle but intentional point of the series was to call-out the neurotic behaviour often found in the "normal" course of life that really is just as damaging and borderline -deranged when compared to the acts of more obviously "insane" people). Each of Tara's personalities have different capacities and skills, some minor, some significant, separate from Tara herself, and each other (Tara is a highly skilled artist/painter - none of the other alters can draw; Buck is left handed, unlike any of the others, and highly skilled with guns; Alice is meticulous and a highly competant cook, whereas Tara is middling, at best.) Go watch this show on Showtime OnDemand. Now. "...[b]ut the worst part of their power is this: they are Toys-in-the-Attic CRAZY! Why, that's not fair. Their madness gives them such an edge over all the other clans. The very idea!" -- ClanBook: Malkavian (1993)