palabravampiress
Bidet of Evil
white avenger agrees: He could be shackin' with Buffy and seein' Faith on the side. Buffy finds out and snaps. Evil Buffy's the next Big Bad!!
As amusing as this idea is (and it really, really is), I don't think it's in-character for Spike.
He's a one-woman man. He falls in love and he falls fast and hard. Like... I don't know. Like a big, giant, heavy rock being flung at slayer speed down a well. It was a neat personality quirk when he was an unsouled vampire. I don't think he's gonna lose that trait just to satisfy our rather depraved urges to see some hot, out-of-character Faith/Spike action. Only if Buffy compltely and permanently rebuffs him beyond even the rather extreme extent to which we were party in season 6 do I think his devotion to her would waver. A one-night stand, a la Harmony, every now and again? Sure. An ongoing arrangement with the same woman -- one known to have poached (or tried to poach) Buffy's last boy (and Xander!)? No. Spike is loyal. Spike's love will drive him to do the impossible (and insane). I don't see him throwing Buffy over for Faith.