Here is how I did it on my Futaba that I fount at this site: http://www.rcgroups.com/forums/showthread.php?t=489099
I found that when I do this, I set it on a switch that I can turn on and off. Then, when you want to use, it, you just turn it on.
The way this works you can leave the throttle at the bottom and then Yaw left or right and that will spin up that motor. If you want you can also throttle up and then speed up the motor you want.
Using my Futaba 9C Transmitter, I was able to program dual rates and exponential as well as several interesting mixes for this model. I programmed Aileron/Rudder coupling, dual motors, and Rudder/Motor differential thrust.
Differential thrust allows you to speed up one motor and slow down the other motor in response to rudder input commands. This greatly aids in ground handling and adds a little something extra to snap rolls and spins.
To program differential thrust, begin by setting up one of your program mixes for dual throttle. Plug a spare servo into channel 3 of your receiver and a second spare servo into one of your AUX channels. Set your mix with the throttle as the master and the AUX as the slave. Set this mix as always “on” by choosing any switch and the “null” position. Use 100% for the mix amount. Verify that both servos move in the same direction and move the same amount from idle to full throttle positions.
Next set up a second program mix with the Rudder as master and Throttle as the slave. Choose a switch to activate this function and use a mix amount of 25%. Verify that the servo in channel 3 moves toward the up throttle position when left rudder is applied and toward down throttle when right rudder is applied. If not, change the mix amount to –25% and test the movement again. (Leave link OFF so that it doesn't try to command the aux2 servo to move also.)
Finally set a third program mix with the Rudder as master and AUX as the slave. Choose the same switch and position that you used for the second mix and again use 25% for the mix amount. Verify that the servo in the AUX position moves toward the up throttle position when right rudder is applied and toward the down throttle position when left rudder is applied. If not, change the mix amount to –25% and test the movement again.
Now we are ready to check the programming on the plane. Plug the right hand motor ESC into channel 3 and the left hand motor ESC into the AUX channel. Turn off the switch that controls the differential thrust mixing. Turn on your radio and plug in your batteries to the ESC’s.
First, verify that the throttle control operates both motors in normal fashion. Hold the airplane securely and move the throttle stick from idle to full power. Both motors should track each other from idle to full bore. Now set the throttle stick to the idle position and switch on the differential thrust-mixing switch. Slowly advance the throttle until both motors barely begin to turn. Now move the rudder stick to give full right rudder command. The left motor should speed up and the right motor should stop. Left rudder command should cause the right motor to speed up and the left motor to stop. Taxi test your model, switching the differential thrust “off” and “on” to test the effectiveness of the 25% setting. If you have a paved runway, you may want to reduce the mix amount to 15%.