Instead of a double bag, think of two bags that zip together. As Phat says, most manufacturers make 'mateable' bags. If Mummy style, they have to be righthand-lefthand for the hoods to work. The problem with mating mummy bags is the zippers get uncomfortable if you want to get frisky.

For the last 10 years, Phriteaux and I have used two semirectangular down bags from Feathered Friends(for cold weather)or one such bag with a zipped-on bottom sheet (for moderate weather - to freezing) Each bag weighs about 2#. We carry one each, then put them together at camp. Using only one bag with the bottom sheet is pretty lightweight, about 2.5#, total. As the temperature approaches 32F, we put the sleeping pads inside, on top of the sheet.