I hope you have your kicker tied with a cross bar to the Merc leg if the kicker does not have its own steering system in use because if not you will get a little steerage from the main when used as a rudder when running under kicker power but not much, that is if you kicker is left straight back and not the primary steering for you boat when it is in operation. If that is the case add a cross connector between the Merc leg and the kicker to solve your problem or some of the other options available such as a hydraulic system for both motors on the same helm/s.
Remember when the kicker is involved in the steering process your boat is not just using the force of the passing ocean water against the rudder effect of the main leg to turn but also the thrust of the kicker to push you in a different direction. Legs on boat motors are not designed to be primary turning devices from the rudder effect like a rudder on a sail boat or shaft drive boat is although they may act as a rudder to some degree and assist with the turn. Primarily in my view at least, it is the redirected motor thrust that is the primary turning force.
I don't think adding a little fin to your main leg for more rudder effect is going to help much if you don't have your kicker doing the primary steering by redirected thrust and may well cause unintended problems.