If you have the 72 mhz 7Cap.... you cant. What you will need to do is either use a Y servo reverser....or what I do is buy a futaba Servo synchronizer(MSA-10) or a JRmatchbox. Using the Synchronizer/matchbox allows you to electronically trim the servos/ set end points etc individually on the ground....and as a single unit in the air.

If you are using the NEW7C 2.4 ghz.... It allows you to use dual elevator under the Ailevator function - HOWEVER....there is a lag between the two channels. Its minor but noticable. Ive asked myself "So whats the point?"

I had the Older 7Cap....and now the new 7C and in both cases I have used the synchronizer.

PS - there was someone on here that was selling used JR matchboxes for a decent price in the for sale section.

Good Luck.