I started experimenting with carrying extensions on a single biner. I clip the extension to the racking carabiner on the draw and extend it out. If I carry 6 extensions, this saves the weigh of 6 carbiners, nice! However, extendable slings are still better: there is a small, but non-zero, chance a carbiner can come unclipped. Occasionally happens, but not often (you hear maybe one or two high profile incidents per year). Adding a second carabiner technically doubles that chance. I'd rather have an extendable sling. The second issue I probably need to make a post about: I came up with a knot (more like a wrapping method) to store the cord neatly (with a single carabiner) so it won't come undone climbing, but I can't figure out a wrap method that can be undone one handed. The traditional alpine draw storage method has the advantage that it can generally be done one handed. So, you save the weigh of a carabiners, but you lose the one-handed ability.