You might look at this idea from Western Mountaineering. A semirectangular down sleeping bag designed to open as a quilt for two. They have a zip on kit that completes the job with making a bottom and holds 2 mattresses.

====== from www.westernmountaineering.com ======

Alder sleeping bag model

5 1/4" Spacing/Continuous Baffles
5" Loft, 25° F
62"/ 4 6" Shoulder /Foot Girth
16 oz. Down Fill
Zips to Summer Coupler
1 lb. 15 oz. Total Weight

For those mild nights when counting shooting stars is all you have to do, then the semi-rectangular Alder MF is your bag of choice! Roomy enough for comfort and warm enough so you won't wake with a chill before dawn. This bag has a 62" shoulder girth and 46" at the foot. The Alder MF has a zipper along the side so that it can mate with mummy bags and a zipper around the foot so that it can be opened flat as a comforter or zipped to our Summer Coupler to be a sleep for two. This bag was formerly known as the "Aspen MF". Nothing about the bag has changed except the name.

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I know where you can get one on good clearance. They were listed in a Westernmountaineering Clearance sale on backpackinglight.com. Last I heard, there was one left in stock, but the deeply discounted sale price may now only be a well discounted sale price.