We have slept under a quilt type bag for many years. The mattresses provide the warmth underneath, and the quilt works on the top. The snag is when it gets colder. Then the drafts at the side make you loose rather a lot of heat. So when we needed something new, we had a dual top bag made for us by a small workshop here in Holland (www.tatteljee.nl). We deciced to go over to Neoairs and Exped 7 downmats for warmer and colder weather repectively. External measures of these mattresses are almost the same. We had a double ultralight nylon sleeve made for these. There is no cold spot in between because the matresses are pressed against each other. At the edges there are zippers to zip a double top bag onto the sleeve. The top bag is about 215 cm wide at shoulder height for an effective bag plus sleeve width of 160 cm (215+105=320). At the feet the bag is rather narrower, however. It is very important not to make the bag too narrow at the feet (not nearly as narrow as with a mummy), because it is of course zipped to the mattresses. I think our bag could be a bit narrower at the top, but a bit wider at the feet would have been nice. My advise is: almost rectangular.
My wife sleeps rather colder than I do, so her part of the top bag has more down than my part. And the weight? Ours is for freezing temperatures and weighs about 1600 grams, including a zipped in silk liner. For another customer the workshop has now also made a more basic model for just above freezing temperatures that weighs about 1200 grams.
So all in all there are indeed efficiency gains in weight (and cost) of about 25% compared to two similarly sized quality solo bags. You cannot really use that gain for a lighter sleep system, however, because the bag needs to be wider than the mattress. So what you gain is the pleasure of each other's company, and more space for the same weight.
From my analysis I also concluded that the gains of a topbag would be relatively smaller for a solo bag than for a dual bag. So for solo use I ordered a PHD Minimus. With a down vest that has a similar temperature rating as our dual topbag system, and it also weighs about half. It is a rather tighter fit, however.