In the past, I've used matching sleeping bags that can zip together. Not top-bottom, but left-right (they had zippers on opposite sides). Works well.

I always figured a two-person sleeping bag or quilt would weigh less than two one-person bags, but strangely, that's not the case. And it's generally easier to pack two small things instead of one big thing, so it's hard to see an advantage to two-person bags/quilts.