thomas3120:
One way I was thinking was an electronic/semi-conductor type of DPDT switch that would accept signal S1 & S2 from the Arduino and signals S1 & S2 from the receiver and with the signal from the 5th channel toggle to switch between the 2 sets of (S1 & S2)'s. The output would go to the S1 & S2 inputs on the motor driver.
That would be a "Quad 2 Input Multiplexer" (74157). That's what the Ardupilot uses.