Doubling your batteries in parallel will increase the available amperage but the board only draws as many amps as it needs. I would check with Erik at imex just to make sure, but I believe you're fine running two batteries in parallel, though I would probably install a 10 or 15 amp fuse in between the batteries and the control board, just in case.

As for charging them together, I would never recommend charging a battery in a tank, but I know you can get boards to charge multiple batteries at the same time.