Bill:

Using an elevon mixer for the throttles - plug the mixer's elevator input into channel three, the normal throttle channel. Then the mixer'a aileron input will go to channel six. The throttle servos will plug into the mixer in plave of the elevon servos. Then the throttle lever will operate the throttles together, and moving the flap knob off center will raise ont throttle and lower the other.

If you have the right mixes available in the transmitter you can plug one throttle servo into channel three and the other into channel six, not needing to use the external mixer.

Bill.