In reply to Sean_J:
Dave Hesleden and I used a single rope + 6mm drag-line on some multi-pitch bolted free routes in Madagascar last year.
The rationale for this was that neither of us likes climbing with a sack (filled with water, food, etc) and that we would use the drag-line for hauling, as well as for the multiple rappels required to get back down.
As it turned out, on all but the very hardest pitches the second ended up carrying the sack — it was just too much of a faff hauling it every pitch, as well as time-consuming (it gets pitch dark there at 6pm in August).
After the first few rappels of the very first route, using the system described by various people in this thread, we were desperately wishing we'd opted for twin 8.5's, but as these were back home in the UK there was nothing we could do but persevere.
Here are a few points arising from our experience, which may be of use to you (and maybe others).
1) Take some spare maillons. If the rap anchors are joined by a leave-behind krab (and most people tend to leave really manky ones) this can get horribly cross-loaded by the knot and screwgate of the rappel rope pressing against it. At the very least take a roll of 25mm finger-tape so you can tape shut non-screwgates on the belays.
2) Even with a 6mm (static!) it was a big effort pulling the ropes down after long raps, particularly if there was any drag. God knows what it would be like with a 4 or 5mm!
3) Because you are always pulling down the rope which will NOT be threaded through belay, it slows down the process of setting up the next rap.
4) While retrieving the ropes at each stance, if by any chance they should get jammed somewhere above (on a flake, say) after the end of the lead rope has passed beyond your reach, you will be left contemplating a prusik on your drag-line!
Hope this helps.
Pete.