BlueJean, Why not just split up the two you have with a switch? I guess four batteries are better than two, but that is two more to buy and two more to maintain and eventually wear out. And then there is the added weight, which is considerable.
I have managed a week (actually eight days) with two people on the two batteries and still had some reserve. We even got weather reports on our 12-volt TV.
No extra lights turned on, no furnace, no exhaust fans -- nothing that isn't absolutely necessary. Oh, I do leave the LP detector turned on. :)