Copy and paste from the [link=http://www.futaba-rc.com/faq/faq-9c-q512.html]Futaba FAQ page:[/link]

I want to set up two servos to operate a single surface, but not use a preset feature like flaperons or ailevators to do so. How can I do this with a mix?
You can use a free mix to set up the twin elevator servos, or any other two-servo combination by simply doing a pmix of the primary channel to the secondary channel at 100% with trim on and link on, offset off and switch assignment NULL so that the mix is always active. Here is an example of mixing 2 to 7:

A. Standard mix from 2 to 7 to work twin elevators:


___ Press Mode (twice if needed) to open Advanced Menu
___ Dial up or down to a free Programmable Mix and press Dial to select
___ Cursor Down twice and turn dial LEFT to activate mix
___ Cursor Down and turn Dial to select channel 2 (elevator)
___ Cursor Down and turn Dial to select channel 7 (Aux1)
___ Cursor Down and turn Dial left to turn Link ON
___ Cursor Down and turn Dial left to turn Trim ON
___ Cursor Down twice and confirm switch position is NULL
___ Cursor Down, pull positive elevator, and turn Dial Left to -100%
___ Push negative elevator, and turn Dial Left to -100%
___ Press End twice to close Menu

B. Turn off default control of channel 7 in Aux-Ch screen so VRC does not affect the second elevator servo.

___ Press Mode Button (twice if needed) to open Basic menu
___ Turn Dial up or down and Press to select Aux Ch
___ Press Cursor Twice and then turn Dial left until reads CH7 NULL
___ Press End twice to close Menu