For twenty years my wife and I used the old "Thermanest" sleep system.
This was composed of two old fashioned full size rectangular thermarest pads, an extra long 20 degree rectangular synthetic sleeping bag and a special joiner sheet.
This system got us used to using one sleeping bag over the top of us long before I heard the word "quilt" used in a backpackery sense, and I used to boast how we were using 0only one sleeping bag for the two of us.
Anyway, the bottom sheet is probably polyester or possibly a poly/cotton blend. It isn't nylon and feels very nice against the skin. The underside has pockets for the top and bottom of the pads, about 12 inches deep. Across the middle is a wide elastic band, sewn down in the very middle.
The bottom sheet is rather longer than it needs to be and has pockets for pillows at the top. These can be stuffed with whatever clothing is handy.
Around the perimeter is a zipper that matches the sleeping bag zipper.
The whole thing is amazingly comfortable and warm. The pads have never, not once, shifted during the night.
The big rectangular sleeping bag provides plenty of space. Drafts have never been an issue with us, probably because we were wise enough to get the extra long sleeping bag. I'm 5'7" and change, my wife is a little taller than I, so we certainly didn't need the extra length, but it provides plenty to wrap yer head or fill any gaps between you.
In cold weather ( about 35F and down ) I will wear a hat of some type to bed though, my thinning-to-non-existent hair doesn't provide much insulation up top.
My wife never wears hats much and is fond of just pulling the bag up over her head. The synthetic bag isn't bothered by the moisture, especially with the heat of two bodies.
This system is so amazingly comfortable we stuck with it as long as we could.
We sleep as good in this thing as we do in our bed at home, and have logged more nights in it than I could count.
The bag is pretty much trashed now, the victim of to hot a dryer and a wife enthusiastic about cleanliness but unfamiliar with synthetic insulation.
The old thermarest full size pads are also heavy, and I've been trying to come up with a system as comfortable but lighter.
So far we have made a Ray Way two person quilt, and quite like it. It's very large and light. It is bulky, but that's OK as it fills much of my wifes pack and prevents her from filling it up with heavier stuff!
I made it to the dimensions Rays says the one he uses is, and I feel it's rather to big for us. Still, with plenty to spare, one of us never gets pushed out from under!
Our Ray Way quilt uses two layers of his Alpine insulation which I reckon makes it about a 20 degree quilt. In the summer it's nice to be able to simply push it off or stick an arm or leg out to regulate temp.
I thought I'd mind not having a zipper to solidly hold it in place but so far it hasn't been an issue for us. Maybe in colder weather it would be.
It would be easy to replicate our original thermnest bottom sheet, and I do miss having that sheet below us. Sleeping nude ( as I often do in warmer weather )on a thermrest pad isn't very comfy. The poly sheet adds a surprising amount of comfort and warmth.
To simplify it I'd probably use snaps around the edge instead of a zipper, and mount snaps in the "draft stopper" of our Ray Way quilt.
By the by, I strongly recommend a sysnthetic quilt, very easy to make. Putting together the Ray Way kit was easy and the materials are top notch. Then again, my wife is a quilter so she knows how to go about this kind of project.
Our arrangement for years has been that she carries the bed - Sleeping bag, both pads and sheet, and I carry all the food and our shelter.
Anyway, our new pads are Thermarest Prolite pads. These are much lighter and plenty comfy, but not as warm as the old pads. For winter camping we go back to our old heavy pads.
Right now I'm simply inflating our prolite pads firmly and tying them together tightly with string, top and bottom. Works surprisingly well!
I'd make a sheet to join them but they are tapered towards the bottom, which I quite do not like. I want a similar pad, just not tapered, at least on one side, so they mate well.
Or a big 35 inch rectangular pad. That would work well for us.
I can't imagine why any couple would want to sleep separately, or want to carry two sleeping bags or quilts. Drafts aren't an issue if the quilt is simply long enough to collapse around/between you as needed, and if necessary, wear a hat.
The Ray Way "bomber hat" is amazingly warm ( I use mine on only the coldest trips ) and only about two ounces weight. It is more difficult to sew than a quilt though.
If anyone is interested I can post pictures of our old thermnest sheet. One photo and a handy feller could probably figure out how to make one.
Edited to add photos simply because I could -
Ray way quilt over tied together pads. I think that orange pad is an early 80s vintage "regular" thermrest pad, but I'm not sure.
That quilt is BIG - That's a three man tent! ( yes, those are life jackets. That was a canoe trip. )
One can see that this is a tied quilt, probably properly called a comforter, making it very easy to make.
Old photo of "thermanest" system inside Clip Flashlight -