Can you not swop the pos and neg cables over on one of them.
Check first in case Im wrong though they are only electric motors after all. If you use a pin you can remove the connections from the plug. 
NO!!! There is a motor in there, but also a bunch of electronics, which will release the magic smoke if power is reversed. The red and black are the power, the white is the signal wire. The signal that controls the servo is a series of pulses, and its the duration of the pulses that tells the servo what position the arm should be in. The way it works is that the electronics looks at the pulse and gives it a value. It looks at the position sensor which is attached to the output gear and compares that with the pulse. If they agree, nowt happens, if they disagree, the motor is driven until agreement is reached.
What is wanted here is a magic box called a servo reverser. I know I've seen them advertised in the past, but don't know if they are still around. The other answer is to build in a reversing link. Electronics is probably simpler, mechanical cheaper.
