My . If you're talking about a 12v motor separate the batteries with a battery switch. A weaker battery will draw down the stronger one, keeping them separate eliminates that issue. You're also able to disconnect the batteries via the switch while not in use which will prevent any drawdown. If you're using a single bank charger you can set the switch to "both" to charge both at the same time(or run off both if you choose).
I was running 2 parallel and when I put the switch in it made a bunch of difference. Also went with 4ga over the 8ga I originally had. The switch and the wire add some costs but not that much in the long run.