Mark, I actually do sing duets with myself, as a gag. There is a preset on the vocal processor on the PA3X that instantly transforms my voice up an octave, and it sounds just like a girl. I can switch from my real voice to the girl and back with one push of a button, or assign it to a foot switch.
Sometimes I "introduce" my female singer guest artist for the night, and then sing "Crazy" (because that's the song EVERY girl that ever sat in with anybody wants to sing). It IS really funny. You'd have to be there. Or not.
I also use that setting when working with my friend Dean Mathis because we have to do his famous hit "I Like Bread and Butter".
DonM