every time you charge to either charge them one at a time, or together
in parallel.

I have never bothered to do that and have never had a problem, just hook up the
battery chargers. Unless the motor is on, the circuit is not complete, I think.
Also, most boats have the wiring built in to handle a 24 volt system - the
wiring actually creates the series connection. You just hook up red wires to
positive and black wires to negative at the battery.
Ronnie

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