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).

I have a rectangular Campmor bag that I use for car camping. It unzips the whoe way around, just like a cheapo Coleman bag, and is big enough for my husband and I to use as a quilt. In cooler temperatures, I'd want something like Ray Jardine's "draft tonsil". There isn't enough blanket there to tuck in and seal out drafts between us.