I wouldn't worry too much about the bleed although there are a few things you can do to cut it down a bit.
Mic placement is the first one. Setting up some sort of baffle between the two mics and sources.
You could also record to a click track and have them record the whole thing playing and singing as a scratch track then go back and redo the guitar seperately and then the vox seperately.
I've done a lot of vocal/acoustic guitar recording in tandem and what I usualy do is forget all about the bleed. Get your guitar track sounding exactly like you want it with EQ, compression, reverb...whatever and forget about how the vocal sounds that's bled into it, then do the same to the vox, then blend them.
Bleedthrough can be your friend and add to the cohesion of your mix.
Play with it and have fun.