I have my charger hooked to one charging both in a 12 volt system. One leg of charger to trolling batteries and one to cranking battery. ProMariner says one leg to one battery only. Never had a problem keeping both charged. Hooking to one battery in a group tied together as 12 volt they will equalize. Ran 4 batteries as 12 volt in big trucks for 30 years with only one charging leg to one battery off alternator. Any time you check them they will be the same as long as nothing is wrong with any single battery.