To run any optional equipment from your second battery, you need 12 Volts. So you'll either need to return your 6 Volt battery and get a 12 Volt, or get a second 6V to run in series with your other 6Volt battery. But then you have three batteries!
My MG has two 6 Volt batteries in series, because at the time, two 6 Volt batteries had more cranking power than one 12 Volt battery. Today, there's no advantage to running a pair of 6 Volt batteries. I replaced them both with one 12 Volt garden tractor battery.