You’re lying if you say you’ve never dropped your belay device and watched it go “tink, tink, tink” all the way down to the base of a route. That’s where you want to get, and you just let go of the one piece of gear that will get you there most conveniently. It can happen to anyone. But have no fear: If you have four carabiners of any shape or gate type, plus a locking belay biner, you can make it to the ground. The double carabiner brake rappel is the best way to descend without a traditional rappel device. It is far more practical and efficient than sharing one device between two people, and it puts fewer twists and kinks in your rope than a Munter hitch does. Plus, you can set it up with the gear you’re already carrying.
Climbers used this setup to rappel in the 1970s, before modern belay devices became the standard. Back then, solid-gate (not wire), oval-shaped carabiners were the norm. While a carabiner brake with ovals is still the easiest to set up and smoothest to rappel with, they’re not necessary; you can rig this with any type of biner: non-locker, locker, wire-gate, or bent-gate. To set up, you need four non-locking biners and one locker (you can use two opposite non-lockers in place of the one locker). However, you shouldn’t use micro-biners, which might not be big enough, or biners with sharp spines, which can damage the rope. Radically bent-gate or pear-shaped biners will suffice if that’s all you have, but they won’t feed as smoothly; the closer to full-sized, oval- or D-shaped, the easier the setup and smoother the rappel.
If you have a locking biner, clip it to your belay loop. If you don’t, clip two non-lockers to your belay loop, oriented with the gates facing opposite directions and opposed (head of one biner is matched with tail of other biner, and vice versa). Now clip two more non-lockers with gates opposite and opposed to the biner or biners on your belay loop (A). Push a bight of both rappel ropes through these two non-lockers (B). Then clip two more non-lockers around both sides of the other two biners and through the bight of rope (C)—make sure the rope runs over the spine of these biners (not the gates). These biners are what act as the brake, and they should have the noses opposed—facing in opposite directions, so one sits on the right side and one sits on the left.
This rappel gives you a ton of braking power, but it doesn’t feel as smooth as rappelling with a normal belay device. To brake, you still change the angle of the rope by pulling it down (and vice versa for speeding up), but the angle change doesn’t correspond as directly to changing the speed of the rappel as it does with a typical device. It feels a little more erratic, so be aware of your speed and positioning.
A downside is that it’s pretty much impossible to go back up the rope. With a tube-style belay device, you can often quickly “hop up” a short section by pulling rope up and through your device before quickly locking off again to gain upward progress. You can’t do this maneuver with the carabiner brake rappel because there is too much friction, but it will help you succeed at your most immediate concern: getting down safely.