Agree with everything said here. I love playing two guitars and voices as you can get very effective results without huge levels of difficulty as long as you keep out of each others space.

You mentioned Joni Mitchell in the first post. If you have access to Blue as an album listen to the interplay of James Taylor's guitar with her guitar or dulcimer and he is always somewhere else adding things. Or some of his albums will often have examples of a couple of acoustic guitars playing beautifully together. I have always thought he is a very talented accompanist whilst being immediately recognisable by the style of his playing.

If you happen to have access to any of the Transatlantic Sessions music (if you in the Uk it is on BBC4 on Friday evening) where I often marvel at the skill that the multiple instruments work together (often not playing anything flashy or incredibly clever but just playing beautifully together with each person allowing others space)