You can't have a Titan be a Warlock as the game is written now, but Bungle can change that any time they like. The only reason for the same character to have multiple faction levels is so that character can buy armor from different faction vendors. Once you get your characters to level 3 or 4 in those factions, you don't need to do any more leveling with them. It may be a bit of a pain, but if people have all their characters grinding reputation levels for all three factions simultaneously, that destroys the storytelling value of factions. At that point, Bungle should just ditch them entirely and have all the armor sold by the Vanguard leaders and Shaxx.