My 1st choice is a two part inner/outer 5 season semi mummy design from Blacks in the UK circa late 50's, "Icelandic Double" (with ratings to -40 ambient by itself). It can be zipped together along the sides to make a 2-3 season doublewide bag. Unfortunately high price and hard to find but very lightweight (800 loft eiderdown).
2nd choice is Norweigan, the Ajungilak (meaning "sleep well" in the Inuit language) Gronland, which is a 4 season down inner/outer combination but the zipper ends up in the middle (unlike the Blacks). On the other hand I added a neck tube/baffle around the neck (duh) area of both - also down filled - which cuts down the drafts when not using the mummy hoods done up tight. Allows seperate movement (can velcro together as one large baffle or each with your own neck baffle).
3rd choice is the Norseman NorthStar 8 sleeping bag rated to -40. This is a heavy duty industrial grade 2 bag in one system whereby you can use the shell/outer bag only which is rated to -10 or keep them zipped together for an Artic rating of -40. The bag is filled with over 8 lbs of Dupont Hollifill. Get a spare second shell and you can use the inner bag to zero. Use them as a quilt with 3 choices of insulation or (using the second shell and inner bag on the bottom) you get a -10 double bag. Gets drafty if you move apart just like a quilt that is tight on the bed and a little undersized. Can usually deal with this however with a little effort.... Volume packed is about the same as a FULL navy duffle bag...
Don't skimp on an adequate foam pad and distance above the ground/floor either! Try a 10cm thick air mattress (NOT inflated by mouth or it'll fill with moisture and freeze as well) with a 15mm Evazote pad on top.
