I'm currently running a campaign where this is happening, sort of. One of the PCs is gradually turning into a secondary BBEG but neither he nor the party are aware of this just yet. Any insights you have or find please share.
I'm not sure how far these two are going to make it. They squabble with each other almost as often as with the rest of the group, and one in particular (Amir the Minotaur Paladin of Conquest) seems to stir up more trouble than is really necessary. As of late, he's been specifically building his character with killing the other party members in mind.
Anyway, if I learn anything of consequence from this experience, I'll be sure to share.