Thanks for documenting the inventory of options.
For about 10 years Jim and I used a Nunatak Back Country Blanket. Custom built to our width and length. Then I added a nylon bottom from the hips to the feet to hold it in place. It worked very well. Beware though, that Nunatak does not fill their bags with adequate down, so you'll need to order overfill if you buy from them.
Nunatak Dual Arc Alpinist is a weird design, in my experience. The footbox is 10-15" too narrow for our comfort, and the torso is 10" wider than necessary. My brother bought one a few years ago and used it only one night on his deck. He and his wife agreed they would not get along if they tried to sleep in it. Jim and I tried it on the living room floor for 10 minutes and agreed. The foot box is just too narrow for us. If you are considering buying one, you and your sleeping partner would do well to find one to try first. (My brother will be selling his, probably at the January GGG at Coe).
A few months ago I sewed a brand spanking new double quilt, which we LOVE LOVE LOVE. Info about the dimensions are in my write-up (link below). The dimensions might be useful if you decide to have Nunatak or somebody else build a custom blanket for you. Also in that write-up is a bit of info about down density of Nunatak and some of the others.
And, if you buy a BackCountryBlanket (or other flat quilt) and tweak it to work for a double, this way of attaching to the pads might help:
Good luck, Amy