I typically do a second, acompanying track when recording, using a capo and alternate chord structure as you describe it. It makes an entriely separate, complimentary voice, especially doing sliding thirds or fifth interval pairs, foor example. I also have a much smaller guitar which I sometimes use for a tenor voice.

Sometimes I have a bad time getting the two tracks in synch though -- my sense of rhythm is so idiosyncratic I sometimes lose the beat of the first track while trying to lay down the second one!!

A