After a lot of market research, we went with a Feathered Friends Penguin Nano 20 plus coupler. It is crazy-cozy-warm, we love the shell fabric, and its specs suited our needs a tiny bit better than the WM Ponderosa or Alder. A custom Nunatak BCB was also a top contender, but ultimately warmth won out over minimalism. We both sleep cold. For warm summer nights when drafts aren't an issue, we're sticking with two separate summerweight quilts.
This was a year ago, mind you, so I don't remember the whole decision-making process. Suffice to say, a spreadsheet was made and the Penguin came out on top in terms of fill-dollar-ounce ratio.
It's bulky, not the lightest, but it is palatial and comfortable and extremely warm. The coupler sheet is awesome; it solves the problem of pad drift--that little gap you get in between two sleeping pads as they shift during the night.
We've also discovered that this set-up (with two cushy inflatable mattresses) makes a great guest bed for those of us too minimalist to furnish the extra bedroom... ;-)
Our quilt-mating experiments did not produce any down (or synthetic) offspring, but we did somehow end up with a human baby. Currently 13 weeks old and 15+ lbs, he no longer qualifies as UL.