A Biner Block (short version - place biner through sling/rap ring, tie biner to rope, clip and lock biner around rope to form loop around main line. Tie thin cord to other line) Would work better.
If your interested in rappel setups, checkout some canyoneering pages. The spend lots of time figuring out different ways to rig raps. www.canyoneering.net, the American Canyoneering Association's page used to have good info, but I can't find it now.