Put the batteries in parallel as you wanted, but install a high power switch between them to enable you to isolate the trolling motor battery. So, when starting the outboard, isolate the switch (open position). After the motor is running, close the switch (trolling motor battery can charge). Make sure the batteries were charged the night before (no high demand from the charging system). When using the trolling motor, isolate the switch again. Make sure the trolling motor is connected across the correct battery (not the main outboard motor battery), so that it does not drain the outboard motor. Make sure to keep the switch isolated until the outboard is running again (if you used the trolling motor so much that it's battery is flat, keep the switch open so as not to kill the charging system). The high amp switches are normally available from auto stores.