If you put two 6V batteries in the battery tray and hook then up in series you will get 12V for starting and charging but if you hook your 6V devices to the (+) terminal of second battery in the series you will have 6V to run them; both the 6V and 12V systems will share the same ground.

Tar tops are still available from restoration suppliers like Antique Battery but they aren't cheap; of course neither are two Optimas.