Bernard, (Stowe, VT)
Bernard, as long as the two batteries remain wired in parallel it is safe to use the charger as long as the output is 12-volts DC. Batteries in parallel, regardless of how many, are simply recognized electrically as one "big" battery. Parallel wiring still produces 12-volts, but with multiple batteries in parallel the capacity to store the current is increased. It's the same voltage (12-volts) that pushes the current. I always recommend to have as many batteries as you have room for and have the capability to keep charged. Remember that the more batteries you have in parallel, the longer it will take to charge the complete bank. Be sure the charger has the output capability to handle the added storage capacity.