I didn't reply before because I have never used the AR7000 (I use Futaba 9caps), but generally speaking ailerons use Ch1 (right) and Ch6 (left), and dual elevator servos use Ch 2 (right) and one of the Aux channels for the left elevator half. One thing you can do is to use the Smart-Fly Equalizer II (http://www.smart-fly.com/Products/Eq.../equalizer.htm) for the elevator and then plug them both into Ch2 while still having the ability to independently adjust the center, direction and endpoint of each elevator servo.

One of the best things you can possibly do is to sit down with your DX7 owner's manual, find the section on mixing and read it through about 3 times, and then play with your transmitter settings with the model sitting right in front of you. I struggled through the 9caps settings on many occasions, went to the Futaba website and searched the threads and always managed to figure it out somehow. This past weekend at the field I was able to quickly program somebody's 6EX with expo and dual rates and put them all on one switch, and he thought I was some kind of whiz. Nope, I just read the bloody manual! I downloaded the DX6 manual from Spektrum (DX7 wasn't available there yet) and the answer to your question is around pages 50 and 51, so I know it's in the DX7 manual somewhere, because it has more features.

Once you "see the light" it ain't all that hard!!