This is true, dual servos for the same control surface is the achilles' heel of the 7CAP/7CAF radio. You have to use some kind of external servo balancer or something in order to make dual servos work.

RC Groups posted a review of the 7CAF/7CAP radio system, and summarized the issue as follows:

"The 7C's big weakness in airplane programming is that it does NOT support twin elevator or rudder servos in any manner. Programmable mixes that don't include link or trim make twin elevator or rudder servos impossible to do properly or safely, except with a y-harness or servo synchronizer. (If you use a mix for 2 elevator servos and use the elevator trim, only the servo in channel 2 is moved; the 2nd elevator would not be trimmed, resulting in roll every time you give elevator. For rudder it would be even worse, as the 2 servos would be fighting each other to move a single rudder)."

The 7CAP is a nice radio system, but considering the Airtronics RD8000 is available for around $250, this lack of support for twin elevator or rudder servos is a glaring omission. I like my 7CAF quite a bit, but eventually I'm going to have to replace it with something more sophisticated.