Here is the best way - get a smart fly equalizer. I just picked one up for my Cap X. You plug two servos into the equalizer and you can match the throws and end points of both servos. This is better than mixing it in your radio or "Y"ing them into one channel. The equalizer will exactly match the servos - it's almost as if you had one servo.

http://www.smart-fly.com/page2.html