ORIGINAL: tacx
You should be able to Y harness your retracts into the gear channel of your reciever. Then use the ''gear'' switch on the upper left hand of your DX7. With a Y harness you are simply sending the same command to the retracts.
I have an H9 P47-150. Each Main Retract has it's own servo. Dual Flaps, Dual Elevator Halves, and Dual Retracts all usually require that one of the servos be Reversed. I do not recommend Reversing Y Harnesses as the Reversed side tends to drift off of Neutral every couple of flights and the Reversing circuit picks up RF Noise and causes glitching in some radios.
I put each Retract servo on a separate channel and mix them using a programmable mixer. If that is not acceptable, use a JR Matchbox. It allows you to connect up to 4 servos to the same channel (2 in your case) and set Neutral, End Points, and Servo Direction for each. Since Retract servos are not Porportional, you may only be able to set Servo Direction.
I use Channel Mixing.
With a DX7, I choose the Flapperon Wing Type and put the Aileron servos on Ch2 and Ch6. You could put the Retracts on Ch5 and Ch7, using a Programmable Mixer to mix the two channels using the Gear Switch as usual.