My research has led me to believe that the FF Penguin is THE solution for a couples turn-key sleep system:http://featheredfriends.com/down-slee...gs/penguin.html
You only have to buy/carry one bag, and you have the option of using it as a single bag or a couples quilt. The removable hoods appeal to me as well. Price rules it out for many though.
Western Mountaineering makes a semi-rectagle bag that zips completely flat that I'd considered making a penguin-style zip-on pad sleeve for, simply because it seemed easier to find the WM used, but unfortunately both brands hold resale value very well.
Many sleeping bags use similar zippers, so even mating across brands would be pretty easy, but finding left and right zip bags that both parties agree on could be tricky, and you are carrying more weight with two complete bags. If you just wander into a sporting goods store with a good selection and start trying zippers (or even bring in one of your bags if you already have one you want to use) you will likely find at least a few options.
I haven't seen any new technology that has changed the down vs. synthetic conversation from what it has been for the last several years.