You have a couple of options. One is the Feathered Friends Puffin, a down bag that can easily be protected from kayak-related dunkings with a waterproof stuffsack. It's a 20-degree bag that's available with a standard nylon shell ($314), or with a water-resistant PTFE-Lite shell (similar to Gore-Tex) for an additional $90. By itself, it's a wide, semi-rectangular bag, perfect for solo sleepers who want a bit more room. But, it can be mated with what Feathered Friends calls the Toucan Ground Sheet. Just unzip the bag, attach each side to the groundsheet, slip in two self-inflating pads, and you have yourselves a bed for two. I'd get the deluxe groundsheet ($53) as it's made of brushed polyester for a soft feel.
The Toucan ought to work with most bags on the market as the number eight zipper is pretty standard. With that in mind, you could get either the 20-degree ($159) or zero-degree ($189) L.L. Bean Summit 3D bag, made with synthetic Polarguard 3D. Both these models are available as full rectangular bags. You could also use a new product called the Sweetie Pie Bag Doubler ($99), an insulated insert that you zip into a bag to make it a sleep system for two.