There are a few ways to charge two 12 volt batteries at the same time. One way is to use a dual battery charger, which will have two sets of charging terminals. Another way is to use two separate battery chargers, one for each battery. Whichever method you choose, just be sure that the positive terminal of each charger is connected to the positive terminal of its corresponding battery, and the negative terminal of each charger is connected to the negative terminal of its corresponding battery. This will ensure that both batteries get charged evenly and safely.