I ran into this last year, AFTER I boutht the 7C. It's really sad that this radio doesn't have this capability to reverse and trim both servos, but that's pure marketing more than anything.

I set up duallys on a Fliton Extra 330 with a reversing Y harness with adjustable trim pot. One elevator half is controlled by the radio, the other by the trim pot on the harness. Works great. I can't remember the brand, but it was only about $30.