Guitar Duets, Adding A More Colorful Frequency Range

One thing that is cool to do is have one guitarist play the song open in the original key, then have the other guitarist play along with you using a capo higher but play in a different key at the same time. Start out at first using a simple song and grow from there.
As an example, one guitarist play open chords G C D,
the other guitarist would capo on the 7th fret and use chords C F G.