If Buffy, Spike, Angel, and Faith were all forced somehow to be in one location together for an extended period of time, how do you think they would interact, considering every specific character's history with one another?
We've seen in Season 5 of "Angel" that Angel and Spike have reached a grudging but genuine respect for each other, and despite the "No Future For You" arc in Season 8, I would feel confident in saying the same thing for Buffy and Faith. What remains to be seen is how Angel and Spike would react around Buffy, as in, would the old "Alpha Male" instinct re-manifest itself, which I believe would be a definite possibility. To avoid any chance of that happening, my guess would be that the safest situation would be for Buffy and Faith to work together, letting Spike and Angel do the same, and both Slayers, for the mutual good of the group, making certain that both vampires are never around Buffy at the same time. Any combined pair of the four would make an excellent team, and I think that that is the way that they should operate on a semi-permanent basis, only all coming together when a situation presents itself where there will be too much fighting of the enemy for Spike and Angel to have time to be jealous, petty, or childish. As soon as the immediate danger is over, of course, each Slayer would grab a vampire (they could flip a coin, draw lots, or play "rock, paper, scissors" to decide who goes with who, but would NEVER make any sort of personal choice. Purely luck of the draw and no cheating) and heads off in opposite directions to do whatever celebrating and/or unwinding they might care to do, anything from playing checkers and watching reruns of "Walker, Texas Ranger" to full out Roman bacchanalia and orgy, with the understanding that whatever does happen is never, ever, discussed afterwards. That's about the easiest way that I could imagine to keep a relatively safe and peaceful environment.
Now, granted, that might sound a bit complicated, and it would still present some problems, but it would serve to keep violence, destruction, and personal injury to a minimum.