In the past every winter my truck sets sometimes for a month or two so to keep the battery up I put a 1.5 amp automatic charger on each battery. Do I need to use two or because the batteries are hooked together would one charger be enough?

 

If your wiring is good, a battery tender is all you need. To big a charger (old style) will boil it dry and then you will need a battery.

 

You just need one, the batteries are connected directly together.