A good way to get two guitars going at once is to capo one of them. So if you're playing in G for example, get the other player to capo up on 7th fret and he plays as if in D. You can use the chart
here to transpose all the chords. You'll find that if each of you is finger-picking through the chords in different positions like that, you'll automatically set up a very nice effect. Just make sure you re-tune the guitar after you clamp the capo on ... they tend to pull things out a bit ... but once they're in tune, it's a beautiful effect.