There are a few options. You didn't mention what kind of pads you sleep on but someone already posted a link to the thermorest site. Assuming you are using some type of pads, I would start with the coupler. It would be easy to make your own. Or better yet, make a combination sheet/coupler. WM has one on their web site (Summer Coupler). It weighs 1lb 4oz (with zipper), almost as much as a ultralight bag and is designed to be used with their rectagular bags. Someone else already mentioned the WM Mitylight. If you're up for the challenge, a DIY quilt for 2 would be very doable. You could have a shared foot box at the bottom or a partial sleeve that fit over the pads at the bottom to keep the system in place and use it with the sheet/coupler. I seem to remember seeing some Jardine plans for a double quilt. I made a quilt with materials purchased from the thru-hiker. It's not that hard.
Campmor sells a few different rectangular down bags but I don't know if they have a fully opening zipper that goes all around the bottom to allow you to open it up as a quilt. If so, that could be an option to either use with a coupler or modify it yourself (after all this is the DIY thread).