Becareful or you will cook one of your batteries. It is rare that both batteries have been drained down to the same level. Most chargers today are smart chargers and will switch to a trickle charge once the battery is fully charged. If you wire two batteries together to charge them at the same time, battery A may be fully charged long before battery B reaches full charge. The charger will not switch to trickle charge for battery A because it is still charging battery B. Hope this made sense. It goes without saying if you do this dont leave them charging all the time.