Actually, the two-rectangular-20”-pads-and-single-bag-as-top-quilt thing that Feathered Friends is doing works amazingly well.
You do need a long version sleeping bag though, because that way you get extra to wrap around yer head and tuck in to fill any gap between the two people sleeping in there. This extra length is probably critical!
I’ve never used the Feathered Friends rig but my wife and I used the similar “Thermanest” system fer nigh on twenty years!
Not once did I ever feel bothered by drafts between the two of us, because I was smart enough to get the long sleeping bag.
Old Thermanest system –
This is the bottom showing the pads. Note that one pad is a Wal Mart blue foam pad!
Doesn't get lighter and cheaper than that. Also doesn't go flat or need blowing up, and is pretty warm. The pad next to it is a 1980s vintage Thermarest and boy, that thing is the ultamate in comfort and warmth, but it is heavy.
The top bag was a single tapered rectangular synthetic bag rated at 20 degrees.
Using it as a top quilt was never constricting at all. But then, I'm 5'7" and 140 pounds, my wife is a little taller with broader shoulders and heavier. Two 200 pound whales might find it tight?
The sheet that joined the two pads was made of a poly / cotton blend and weighed about a pound. Naturally there was a zipper on the sheet that matched the bag.
Over time, the synthetic bag died and has little of its original warmth and loft left, while retaining the original weight. Time to make another.
My new system was an attempt to re-create the warmth and comfort of the old system at a lighter weight, and was entirely successful.
We did try just draping the Ray Way quilt over us as per Ray Jardine practice, but it wasn't very warm. I simply used string to bind our two tapered thermarest pads together, and that sorta worked, but it did not provide the cozy draft free "bed" that a full joiner sheet does.
Indeed, the bottom sheet can be made lighter, but I was going for warmth and cheapness, and had the fleece handy. I did use velcro instead of a zipper for simplicity and light weight, but a full zipper will be warmer.
Do not underestimate the warmth that the joiner sheet adds to this system. While it does not have any loft of it's own any trapped air space adds to the warmth and it seals out the drafts very well, and is a much nicer surface to sleep upon than a plastic mat. This last is important to us because we sleep nude in all but freezing weather.
We don't need to wear clothing to bed, this system is warm enough without it for most 3 season camping.
Also, a very real bonus is the shared body warmth! You just don't get that with two separate sleeping bags.
Our Thermanest system was heavy, but it was still lighter than two bags and two pads, and got us used to thinking of a single bag open as a top quilt over us with "just" the pads for insulation below. Twenty years ago I thought we were being pretty slick and high tech and all that!
Trust a couple that has been using such a system fer a very long time, it works very well, and for us was as comfortable as our bed at home.