Nick,
One of the biggest issues I have when mixing worship bands, etc. is with their guitars and getting them to sit nicely in the mix AND play well together.
I find that, often, the guitarsits are playing very similar chords, etc. which makes everything blend together. Do your guitarists do lead, rhythm, etc. well enough to have separation on their own? If not, there's no way you'll get the separation you're looking for.
How do they sound just running through their amps? If they're not on amps, how do the sound solo'd at the console? If they're not doing different stuff, it'll just be one big pile of mush.