So the house will stay at 72F, even if the power goes out? That's excellent!
If there's a warmer room, I'd move the entire tank into it. As Callistra said, you're better off keeping them in the larger tank, and monitoring the temperature, as Isochronism suggested.
To help retain heat, if needed: You can wrap the tank with aluminum foil, then wrap it again with towels or a blanket to provide extra insulation. Cover the lid too. (The lid/top is a large potential source of heat loss.) Make sure the bettas can breathe though!
If heat loss continues (which I don't think will happen, since the house temp is 72F): You could always heat some water with the stove, and do small water changes. (Obviously, don't make it too hot, and don't add it rapidly! You don't want to shock the fish with a drastic temp change. I suggest using a thermometer to monitor the temp of the heated water, and keep it within a few degrees of the tank temp.)