I personally would use 10 gage as a minimum, or one size larger than the
trolling motor.

Yes you can charge both at once. Just remember that charging will be just like
the use twice as long.

Mike
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I plan to buy two 12V trolling motor batteries and hook them in parallel for
use with my 12V trolling motor and Q-Beam spot light. What gage wire should
I use to connect the two batteries? When I charge the batteries can I
leave them connected in parallel and charge them both at the same time? I
do not use an onboard charger