You can charge parallel, that's the on board works. It is best to charge each individually. You can unhook the parallel wire between the two pos connectors then charge each battery individually. You want the two batteries as balanced as possible. Charge both individually, let set for 5 min after the last bat is charged, then check with voltmeter. The voltages should be close. While charging with an external charger, disconnect the battery from the on board by turning off batt switch located under the sink. When batteries are parallel charged, the more unbalanced, the shorter the life. If cutoff on the charger is 14 volts, that can be 16 on one and 12 on the other. They will try to balance on discharge, but the more cycles, the more out they get. It's a great idea to balance them every year.