All of the Duet Set carols have a demonstration video. You can listen to O Little Town of Bethlehem, Away in a Manger, While Shepherds Watch Their Flocks by Night, Hark! the Herald Angels Sing and Joy to the World.
The duets are written in bass clef. They fit with the red and orange sets of music. The melody moves up an octave and the duet partner joins them at the piano. The parts could also be played on the cello or other bass clef C instruments. The sheet music is available for purchase here.