I have been trying to find the "canonical" post on this topic without success - each post has its own take on the issue. And I understand that opinions and preferences abound - but I'm looking for sensible principles to apply (and resultant recommendations). Perhaps the most reputable discussion I found is on Mountain Project.
But if I were to summarize what I have learned I'd say that:
- You need two devices for redundancy, separate from each other as in this video:
- Petzl Microcender seems to be a crowd pleaser, among other things, because it is toothless, and doesn't fray or peel on the rope. However, it has been discontinued. Same goes for the Silent Partner and the Soloist.
- What is the (or some of the) preferred secondary device? On the first image above, the secondary device (Handled Ascender) looks quite bulky... Should a toothed device like the Petzl Micro Traxion be a favorite option? By using it as a secondary device, you get all the benefits of a toothed device, but protect the rope since it will only get engaged if the primary device fails.
- Is it best to use a static rope (less elastic excursion; less problems with toothed devices) with two strands equalized via, for example, a figure 8 knot at the top - one strand for climbing; the second strand as an escape belay or lowering (gri-gri) rope? This is probably shortsighted, giving priority to the rope over avoiding potential spine injuries with falls on static ropes - although possibly not all that likely...