Ok, so if you have already put all the lyrics into the karaoke track either vertical or classic
just right click > add track > Vertical for scrolling, Classic for still (best).
Then where the duet singer lyrics start, click the first lyric then go to the last lyric
in that verse > hold shift key and click it, this will highlight all the verse > Edit > cut
> put the time bar on where the verse start on the second track > Edit > paste.
Now remove the lyrics out of the 1st track by double clicking on it then text box
will come up and delete the verse you just put in the 2nd track.
Now carry on with the other verses, you can also add another track in another color
for when they both sing together, and again just adjust the position of the text on the screen so they don't overlap.
Try this :
http://www.margosmusic.com/kf/Duet.wmvAlso here but did not change the sync bubble color :
http://www.karafun.fr/forum/share-your-karafun-experience/text-in-different-color-singing-duet-t2109.html 