If you are set on MYOG, that's cool. If not, check out the military surplus poncho liner. A thin, very light, square quilt blanket. Just shell and a bit of synthetic fluff, nothing fancy. They are very roomy, I assume they could fit two bags. Definitely two people could sleep under one buy itself with plenty of room. They weight right around 1lb.
They make great underquilts for hammocks as well when it gets around freezing.
Edit: oops, marc beat me to it.