In reply to climbwhenready:
Yes a tricky one the problem comes from dealing with your casualty so that his full weight doesn't come onto you and double weight a prusik, this is probably how I would do it but they will be others, many probably better;
1) Rig best anchor you can.
2) Take an 8' sling, fold it in half and tie a small loop at the mid point. You should now have a sling that is made up of two large loops and one small loop at the middle (creating a Y hang sling).
3) Clip one of the large loops to your ab loop on your harness.
4) Thread the rope through your belay device in the normal way and clip it to the small loop. This will make the belay device about 2' away from you but still well within reach.
5) The other long loop should be hanging free. Clip a crab into it and clip it out of the way (some clip into the belay device crab).
6) Tie a normal prusik to the rope and clip it to your abseil loop.
7) Set off down rope.
8) When you get above your mate stop. Tie a clove hitch in the rope below your prusik and clip it to your abseil loop.
9) Let your prusik take your weight.
10) Clip the spare loop to your mate. You will need to get him off whatever rope he is attached to. Clip a short quickdraw from your harness to his, this will need to be with screwgates. (If he is hanging this gets a little complicated and you might have to start getting involved with Yosemite lifts and mariners hitches but lets brush over that and say that he is on a nice ledge and can get his weight off the rope).
11) He will now hang at the same level as you (this is not overly comfortable but as you practice you will start to alter the lengths of the loops on the Y hang to suit but lets gloss over that and keep the loops the same length).
12) Release the clove hitch and carry on the abseil until you reach the knot. Directly below the knot tie an over-hand knot on a bight.
13) You will have to get it past your prusik first. Let the prusik take the weight and then tie a back-up clove hitch a meter or so down the rope and clip it to your abseil loop.
14) Place a second prusik above the belay device and clip to your harness, place extra turns on this one as two of you may be be weighting it.
15) Release the first prusik and remove it from the rope and place it below the knot.
16) Abseil down a little bit more until you weight the top prusik. Check that it is locked and check that your back-up clove hitch is still in place.
17)Remove the belay device from the rope, they should be hanging from your harness via the short quickdraw (Point 10)
18) Clip your friend to the loop you have made in the rope (Point 12) using sling. This should be tied to the loop by way of a tied off Italian hitch onto a crab on the loop and pulled tight. The significance of this is that it can be released under load.
19) Remove the belay device, your casualty should be hanging off your harness via the short quickdraw and re-attach it below the knot. Tighten up the sling in Point 18.
20) Release the top prusik until the weight comes back onto the belay device.
21) Release the sling from Point 18.
22) Release the clove hitch.
23) Abseil to tea and medals.
Post edited at 15:16