I love this setup. I installed it this spring, and it is working great.
The "Add a Battery" has an isolater that automatically favors the motor battery. Once that hits 12.5v, it will start charging my stereo batteries. Unlike the Perko (Bat1, bat2, all, Off) no switching is necessary to get both banks charged. And if for some reason one of your banks goes bad, you can combine the banks to get enough power to fire up the boat.
Also on a Perko, if you forget to turn the dial, you may end up with a dead battery.
I also did not use the switch. I run only the 7610 Relay unit, the switch is still in the box. I am running one 12v battery for the motor and (2) 6vís for my stereo. I have 3 amps at around 2500 watts total. Batteries are always full or close to, once I am off the water and test them.
Have a great summer!