In reply to The Ex-Engineer:
> I've certainly considered the question of whether abseil ropes running over a double strand rather than a single strand of thin cord would improve matters (as just discussed by "older"). I'm also not sure if it helps, but it can be easily accomplished (if the single strand is long enough) just by tying a bowline on the bight in the cord. <
> I've gone for carrying high-strength 6mm stainless steel rings <
I may be becoming obsessed with minutiae.
However we are probably too pessimistic about the value of 2 strands of different length on one sling. Bodyweight is taken by one strand first which alone will melt through after X amount of any sawing action, then the second strand will take X sawing action before breaking: ie overall twice as long before catastrophe. As Tesco would say: almost 2 for the price of one. Less, if any, advantage if bodyweight on both strands together and melting simultaneous.
Not sure about the sheath of tubular tape method. It probably only provides protection until a nick is burnt through it, after which most melting action is back on the sling. A glued or sewn leather tube sheath should work well. (I used to have a shop-bought, spliced, hawser-laid belay sling with 2 of these, presumably made to prevent cutting on sharp edged spikes.) This or your steel ring might be best.
Incidentally I'm struggling with how to tie a full weight ab sling with a bowline on the bight through the hole, though that's just me.