Here's a solution that might work for you. My wife and I do this.

Use a sleeping bag on top (opened up so it is a flat quilt) and a piece of uncoated nylon on the bottom.

I sewed a zipper to the nylon so it can be zipped to the top sleeping bag just as you would with a second sleeping bag (as described in some of the other responses).

You could simply sew the bottom nylon to the sleeping bag if you wanted to save the weight of the zipper.

This combo of bag and nylon bottom weighs much less than two sleeping bags and is quite warm. My Feathered Friends bag weighs about 2 lbs and the zippered bottom weighs about 8 ounces so the combo comes in at about 2.5 lbs total for the two of us.