Since your going to replace your Tx and Rx consider a computer radio like the taranis. Each of your rudder servos can be on its own dedicated channel as can the ESC for each motor. Using the built in mixing functions you can "tune" the setup so as to get the effects your looking for plus you can do things like both rudders moving in opposite directions to give a brake effect. Setting up the mixing looks difficult but once your into it, it follows a logic.

looking at flanking rudders you will need another servo working these and this can be either with a another channel or a Y connector working all servos have a look at "Google" how flanking rudders work hope this helps