Keith,
Cut through and tape up the +ive lead on the foot of a standard Y lead
Then
From the retract battery to a standard switch harness to the modified Y lead to the retract channel on the Rx.
From the other leg of the Y to the retract servo.
What you are doing then is powering the servo from the seperate battery (JUST MAKE SURE THAT YOU MODIFY THE Y LEAD BEFORE YOU PLUG IT ALL IN)
All the other channels and flight battery are connected as standard.
Try it all on the bench first. You'll see that if you unplug the retact battery to simulate a dead one, all the flight controls still work.
If you still have it have a look at the wiring on the twin mustang, piece of cake
It has to be a Y lead, it wont work with the block type leads where both servos go into a block
Hope that helps