If I put the direction control input (aileron stick) full to one side, both wheels start to turn in different directions. That´s good.
If I then move the speed control input (pitch stick) just a little bit forwards or backwards, the wheel that would be on the outer side of the turn immediatly goes full speed forward or backward.
I hope, my explanation is understandable. If needed, I can make a video of the problem.
RPi 3 with Navio2
Sabertooth 2x12 motorcontroller