The 9C TX has that mix already setup in the programming. The following is copied from the Futaba FAQ section on their web site.

Question: What is the easiest way to do twin elevator servos?

Use the ailevator function, designed for just this purpose.
Plug servos into 2 and 8
Go to AILEVATOR
Make the function active
Test your elevator movement. If one is going the wrong direction, reverse the % for that servo (ie -100%).
IMPORTANT READ ME: Do you want your elevator servos to also act with your ailerons? If NOT, you must reset the AIL3 and AIL4 numbers to 0.

The FAQ page can be found hear
http://www.futaba-rc.com/faq/index.html