When riding double we take a light weight fiberfill and a down filled that even through it is a square bag is rated to below freezing temperatures. If it is hot the thin bag goes on the top and when it is cold the down-fill goes up top.
Now the trick to sleeping comfortably when it is getting rather chilly out there. Don't use a air mattress, as the things really have no insulating ability. Get those self inflating foam filled mats and the best are those which are nearly an inch thick.
Then the coupe-e-gras. Find a rock about as big as your head and stand it up beside your campfire. Get it good and warm. If you spit on it and it sizzles it is just a little too warm ... ;-) Then wrap that rock up in a towel and tie it with string. Wrap that string around several times as you don't want that hot rock to escape. Then throw that in the foot of your sleeping bag. It will still be warm in the morning. That makes a bag which is good to freezing ... good to at least Zero.
You know it is cold when you and the wife go for three rocks. Two for one each of your feet ... and another between you for your backside ... :-).
Bye the way ... the run around the tend absolutely naked so that when you get back in the sleeping bag it seems nice and warm ... doesn't work. Don't ask me how I know ...
My theory is ... in the future everything will double as an iPod Dock.