Thomas, Depending on the size of yourself and your companion, consider carrying one large bag. I did this once and it worked great. Here are a couple choices for applicable bags:
1. Feathered Friends suggests using their Wide series for two campers: "For those who want to use one bag for two people". These bags have a 64" circumfrence at the shoulders.
http://www.featheredfriends.com/productlist.aspx?SubCatId=3&CatId=1&selection=3
2. Montbell SuperStretch bags (either down or synthetic) fit one 6'4" occupant, or two smaller people. Shoulder circumfrence is 75" max(that's not a misprint); rest of bag stretches accordingly.
My #7-Long down bag fits me and my GF (120kg total) easily, and weighs 510 grams. That's like each person carrying a 9.5 ounce sleeping bag, but more fun.
https://www2.montbell.com/america/asp/products/Spg_itiran.asp?cat=1101
Note, the "Alpine" option on the odd numbered bags reduces the width for a lighter bag; I suggest the the non-alpine ones.
3. Snugpak synthetic bags have a full length baffle you can extend to get several more inches of length. No stretch system though.
http://www.snugpak.com/20_outdoors/21_05_micro.htm
On a tangent.. if you sell the thermarests you mentioned and get 2 full length Montbell UL pad sets and you will save another 5.7 ounces and gain the attachable, inflatable pillows. The ULs are same thickness, and solid foam, not die cut like the thermarests.
