Guessing that you have at least two house batteries, I'd sy you have a battery problem if they won't run the furnace all night for one night. Or maybe you have a battery charger problem.
But adding batteries is good too. If there is not sufficient space in the existing vented battery compartment, you will have to use either sealed batteries (e.g. AGMs) in an unvented space or find some way to vent another space for the extra batteries.
As has already been said, if the existing batteries are in poor condition, you don't want to wire two new ones to them - you just end up with 4 poor batteries instead of two. I'd replace the existing ones first, then see if I still needed more.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL