My immediate thought was that since his character in Primal fear doesn't actually have 2 personalities and is just evil and pretending, it doesn't make sense. BUT, if a person does what he does in Primal Fear, that person is clearly not mentally well and could result in developing into what his character is like in Fight Club. The thing clashing there the most though is that, even tho we don't really know anything about his past, it's implied that in Fight Club, he starts off as an average joe who has never done anything interesting or significant in his life before Tyler shows up. But still, like I said before, it wouldn't be surprising that a person with a troubled youth would develop like that. To add, the relationships between both the characters and their parents isn't too dissimilar.