ORIGINAL: Tall Paul
I've done differential steering with twins using a Futaba 6EXA transmitter, which has a knob useable for mixing the two motors.
Cumbersome and not really good for ground handling, I used it to see how rudder authority was altered with one motor off.
On the DX-7, you may be able to get a mix with the throttle channels and the rudder stick, for water manuvering.
Hmmm.. I am just not sure how to run two motors on two channels. I gather I would need to ESC? Would that be right?
Might try and stick with the idea of a better water rudder as her flying characteristics are fine.
Cheers mate
-J