The single most important thing (musically) you have to do with 2 or more guitarists is stay out of each other's way. If you end up with 2 guitars playing the same thing, often it will sound worse than if you only had one guitar playing. This isn't nearly as easy as it sounds, especially if both guitarists have similar styles, however if one of the guitars is an acoustic it becomes less of an issue, as the acoustic has such a different sound.
One other thing to keep in mind also is that music needs to be able to breathe. One thing I see alot in worship music is that too often every musician plays every note in every song. That tends to make songs boring and, worse yet, very muddy-sounding. I'm still learning when to play (and when not to,) but I've noticed that often less is more.
At
Port City Church, our typical lineup is one drummer, one bassist, two electric guitarists, two female vocalists, one keyboardist, and one male vocalist w/acoustic guitar.
Tom