Over the last few years a number of hard ascents have been achieved using one sleeping bag between two climbers, using it like a blanket. This is a very interesting concept and primarily stems form climbers wanting to carry only one rucksack, carried by the second, and also from ultra light back packers in the US, who use two person ‘blankets’ (check out Ray Jardine’s web site at http://www.rayjardine.com for more on this). Both climbers wear all their clothes (generally they have synthetic parkas or down jackets), and sit or better still lay side by side and lay the bag over each other, tucking it in under their sides. Both climbers need to lay still on their backs, lying close together, and for the greatest performance they will need to spoon together (great way to make friends). Having spooned together many times with cold partners the warm generated is considerable.
Thoughts on making it work include making sure the bottom of the bag will take both climbers feet (a synthetic bag works best as they are generals oversized), and having some way of joining both climbers sleeping mats (need full length mats). If you plan to use this concept I’d also recommend sticking some loops on the bag so it can be secured underneath you with slings or cord (at the knees, waist and shoulders). If you really want to go to town it would also be possible to sew a triangle of material with two matching YKK zippers that would zip into the bag to form an enclosed ‘top bag’.
This concept works better than it sounds for tents bivys, or were you can lie down, and is better than nothing for sitting bivys.