You can try to use two servos from differental manufactureer. I had tried before in my TLT-1. I use MG995 at front and Sanwa(?) 577 at the rear side. Front and rear side turn at different direction. If you use the servos from the same make, it certainly will turn at same direction.
There are other methods from other C hing, some of them use the third channel to control. But it is not good, I think.
