Any 12 volt charger will work just make sure it has a shutdown feature that will not over charge the battery. 2, 4, or 10amp, most newer smart chargers shut down to a trickle when the battery is charged.
It is already fitted with a battery charging coil and rectifier for charging its own starter battery.
IMHO the best bet would then be to connect the trolling battery to the starter battery via a VSR (voltage sensitive relay). That would allow the starter battery to be recharged as the first priority and then it would automatically connect the trolling battery for charging once the starter battery is recharged.
A diode splitter is another possibility but they drop the volts by about 0.7 volts.
Manual connection ( ie with a switch) after allowing the starter battery to recharge is also a possibility.
You don't say what model you have but those with power tilt and trim have a 9 amp stator coil and a regulated rectifier, those without have a 6 amp stator and an unregulated rectifier.
Remember the electrical power from the engine is limited so you may not fully recharge your trolling battery. You should not leave it partly discharged so you may still have to remove it for charging.
Unless you run the main motor a heck of a lot more than the trolling battery, if is futile to try to charge it that way. It just won't put in as much as you are taking out. OK, so every little bit helps if you want to look at it that way.
