If you have a large group (4+) and want to cut down on the wait time at a rappel, you can isolate the strands of your rappel rope so that each strand can be used independently. That way Person #2 can start getting set up while Person #1 is still rappelling. Another reason to isolate the strands is to allow people the choice of rappelling single strand or double strand.
To isolate the two strands:
Thread the rope through the quick link or rappel ring so that the middle of the rope is near the anchors and both ends reach the ground.
Attach a figure 8 to both strands the same way you would attach if you were going to rappel down.
Clip a locking carabiner through the small hole of the figure 8 and into the quick link on top of the rope.
This method allows large groups (4+) to rappel more quickly. It can also be helpful if there are novices in the group because if the novice gets hung up, the guide can rappel down the other strand and help the novice get unstuck.
This is a complicated rappel setup in which several things could go wrong. If in doubt, K.I.S.S. it (Keep It Simple Stupid) and just rappel double strand.