Hi, big joe,
I played half of a duet for about 13 years. I think it's the hardest group format in the world. No room for error one either person's part -- it SHOWS when one of you is off. You can neither say, "Well, that's my interpretation (tonight, anyway)," as a single act can, and there's no one covering the error, as trios etc. can. Duets demand real concentration on every song.
Both of us played guitar as our primary instrument. I took up the tenor banjo for the occassional song, but usually we traded out playing lead/rhythm. My partner was a much better guitarist than I, but he used a flat pick and I fingerpick. We'd capo at different positions and play different progressions to get a different sound. In other words, as Gary T lists above. We mixed up our rhythms in performance, making and effort to switch them around. And we worked like dogs on our harmony and timing.
And I gotta say it was worth the effort. I learned a lot, just working up songs to fit our format, and working with our format to fit the songs.
When you get it right as a duet, you know it, and it's great.