To simul-rappel, the technique goes as follows:
- Prepare to rappel as usual, passing the rope through bomber bolts;
- Tie knots on both ends of the rope;
- Attach your personal anchoring system (PAS) to your partner's PAS or belay loop (or remain tied to him in any other way);
- Set the gear to rappel from a single strand of the rope simultaneously with your partner, who's going to rappel from the other one;
- Weigh the rope and be absolutely sure neither you nor your partner has its weight placed on the PAS, but on the rope (standard procedure: never untie from something that's hard to untie from). In other words: make sure you're both tied to the rope and balanced only by your weights;
- Untie both PASes and rappel down singing a beautiful song, preferably in a sonorous language like Italian, Portuguese of Russian (German and Polish songs may cause accidents);
This method is faster than usual rappelling. Many people use it frequently, and even more people have used it at least once. I like using it a lot, and one of these days a friend came to me saying that another climber got freaked out after hearing he used simul-rappelling in a daily basis. According to this guy, simul-rappelling is more dangerous than rappeling.
This got me thinking: imagine the belay is bomber and your rope is trustworthy (if those are not matched, then rappelling won't be safe regardless of the method). Then why would simul-rappelling be more dangerous than rappelling? I can only think of one situation where this is true: the weight difference between the climbers is considerable plus they're not attach to each other (as when rappelling from opposed sides on a spire). Overall, this is easily solved by remaining attached to your partner - actually, sometimes going down is only possible if simul-rappeling (like when one of the climbers is so light that the rope won't run through the brake - this happens sometimes to children).
Does someone have a clear, precise reason for simul-rappelling to be more dangerous than rappelling? Since most climbing accidents happen during the rappel and many of them are due to the fact that one of the climbers didn't actually pass his/her rope through the breaking device, I'd say simul-rappelling is actually safer than standard rappelling, since both climbers need to be balanced on the rope (using their braking devices) before lowering themselves.