Mrs C and I use two single airbeds with a fitted double sheet joining them together, then our usual domestic duvet and pillows. We also have a 'fleecy' (in that it looks a bit like sheepskin, not micro-fleece) fitted sheet which goes under the domestic one.
Advantages. When she gets up in the night, I don't hit the ground! You don't get the gap which tends to happen when you join two sleeping bags together. You can inflate the airbeds outside the tent.
Disadvantage. Takes up a lot of room, but we have a Kangoo, so space isn't really a problem.
The problem with joining two bags together is that the open area at the top is 4 times the size of a single one. My parents gave up zipping their sleeping bags together as it was just too cold (these were the rectangular sort, not mummy style).
S