Good question, and I surely don't have the definitive answer except for (d) below, which is even more expensive than most couple's bags. Here is what I have tried:
(a) Western Mountaineering semi-rectangular bag [was the Aspen when I owned it, but I think it's been renamed] and Summer Coupler. OK down to about freezing. Could go lower with a stouter WM bag but mine (Sequoia - 20 years old) didn't mate with the coupler.
(b) Nunatak Back Country Blanket. See my review of this at http://tinyurl.com/7slg7h Works in 50 F+ weather and down to freezing "only if the users are close friends neither of whom is a blanket hog."
(c) Tim Marshall couples-specific bag. A fellow BGT member very kindly sent me his. Similar to Nunatak's couples' quilts but shorter and narrower. Used at 20 F, with each of us wearing a down sweater. A bit cramped.
(d) Two Back Country Blankets, discussed in the addendum to my review cited above. The best option but REALLY expensive unless you're as lucky as I was in scoring two at bargain prices.
This is a gear issue highly dependent upon how each of you sleep - hot or cold, close or slightly apart, dead to the world or restless all night.
If you find your solution, please advise!