charge 4 batt with three outputs?

[dupsta]

Question on multi charging 12v batt
I have an on board charger for my bass boat. I use 2 marine deep cell batt for my trolling motor and one starting batt for my outboard. My new GPS/Sonar electronics will drain the starting battery. I want to install a fourth smaller (lawn Mover type) 12v battery.
My on board charger currently uses all three outputs for three 12v batteries. Can I split one output so that it will charge Two 12v batteries, the Starter batt and the smaller 12v batt I want to isntall?

[FAST FRED]

I would move the GPS to the trolling motor batts , as the demands of a GPS are minor.

Otherwise tie the two deep cycles together (similar batts will have no charge problems) and use the 3rd output for the GPS.

FF