This is a pretty tough situation. As has already been pretty well covered here, if the rope is running over a slab or large roof (and you don't happen to have another rope handy), you're pretty well boned, as there is no way you're going to descend a loaded rope running over the rock. But let's assume that we're fortunate today, the rope is running free from the anchor to the fallen climber, and we're able to descend safely with twin prussiks. Here's what I would do then:

1. Descend until your prussiks are a foot or so above the fallen climber's belay / rappel device.

2. Tie both yourself and the fallen climber in short with fig-8s or butterflys with the free rope hanging below, leaving a good few feet of slack. There are probably a couple ways of doing this, but the easiest may be just tying one climber in short to each of the lines (i.e. counterweight the line). This will act as backup if anything goes wrong while doing the rest of the rigging.

3. Use a long cordalette and HMS biner (that you hopefully didn't decide to leave on the ground) to construct a PMMO hitch on the lines above the prussiks you used to descend. The biner side of the PMMO is clipped to your belay loop but not weighted yet.

4. Girth hitch a sling to the fallen climber's belay loop, and clip it into the PMMO HMS biner, shortening with knots as necessary so that there isn't more than a few inches of slack.

5. Before weighting the PMMO, rig your belay / descender to both your belay loop and the sling connecting the fallen climber to the PMMO (of course, you can't thread the rope yet, as it is still weighted, but you want the belay device carabiner clipped through both your belay loop and the fallen climber's sling, as it would be difficult to rig once the sling is weighted).

6. Gently apply your weight to the PMMO hitch by sliding your descending prussiks down (which should only take an inch or so if the PMMO is properly rigged).

7. Gently apply the fallen climber's weight to the PMMO by sliding him down the rope a few inches (which may not be hard if his third hand is rigged below his device. If rigged above it, you may need to cut the cord in order to unweight it). You should now both be hanging from the PMMO hitch, you attached by your belay loop, and the fallen climber by the sling running from his belay loop. You are now both dependent on a single prussik knot, hence the tying in short in step 2.

8. Now that both climbers' weight is held by the PMMO, you can remove your descending prussiks from the line. Your belay device should already be clipped into both your belay loop and the sling from the fallen climber's belay loop (step 5). Now that the line below the PMMO is unweighted, you can rig your belay device onto the line (including a third hand prussik backup).

9. Remove the fallen climber's belay device and third hand backup from the line. Now you should be ready to use the PMMO to gently lower both of you on to your belay device, making sure your third hand backup engages properly.

10. Remove the (now unweighted) PMMO from the line, do anything else needed to prepare for descent (rigging a chest harness for the fallen climber, etc), and finally remove the backup short tie ins from both yourself and the fallen climber.

11. Rappel.

Sounds like it should work to me. Any thoughts?