Charging two 12-volt batteries together saves time, although the process still takes longer than charging just one 12-volt battery. However, you need to wire the two batteries together before you begin charging and it needs to be done in parallel, not series. Parallel wiring means the connection between the battery charger and the two batteries remains at 12 volts, but if you wired the batteries in series, the batteries produce 24 volts, which is more than the charger produces, meaning the batteries are unlikely to charge.
Connect your two 12 volts batteries together using two battery cables. You can charge your batteries together, once they are wired in parallel.
Attach the clamp on the end of one of the battery cables to the positive terminal of the one of the two batteries and then attach the opposite end to the positive terminal on the other 12 volt battery. The terminals are clearly labelled "+" for positive.
Attach the clamps on either end of the second battery cable to the negative terminals of the two 12 volt batteries. The terminals are labelled "-," meaning negative.
Attach the clamp of the end of the positive battery charger cable onto the positive terminal of one of the two batteries. The battery charger cable, or the clamp handle, is coloured red.
Attach the clamp on the end of the negative battery charger cable to the negative terminal of the same battery as you just attached the positive cable to. The battery charger cable, or clamp handle, is coloured black.
Set your battery charger to the charge rate you want. You probably find it has a slow charge rate such as trickle charge, or a fast charge rate such as boost.
Plug in your charger to the power supply. Turn on the charger. Your two 12-volt batteries get charged together, but it may take longer than charging just one. Turn off your charger once the batteries are charged.
Disconnect the battery charger cable clamps from the terminals. Disconnect the two battery cables from the two 12-volt batteries.