Bose is exactly right!
For us to add duet requires several things:
1. A way to enter two separate lyric word streams sync'd to the music.
2. Settings controls for the second singer (font, point size, style, text color, sweep color).
An added bonus would be to provide a way to allow color both singer's lyrics a 3rd color when they are to sing together. However, technically this could be complex and I'm not sure if the singers are better off seeing a color change instead of just singing to their color.
We have this capability (1 & 2) in Karaoke Pro. To bring it down to KHProducer will take some time. We need to get 2.0 released first. Then of course there is adding graphics and editing the placement. Editing really will be required before we would consider adding duet.