i beleive you are going to have to do a mix throttle to rudder. Rudder as the master and throttle as the slave, as I have not done this before if you have the manual on page 31 and 32 it shows how to set up mixes, the manual is online.
@chrisvdv I am also having issues here and I was looking at the shots above compared to the dx9 guide which I had been following. It looks like you have an extra "Mix 0" for Thr>Ax1. I tried adding that and it just made Aux1 stay at -100% when I adjusted throttle.
What I see on my DX8e is with the below mix I can raise my throttle to halfway and Aux1 mirrors it. Once I go full throttle, Aux1 stays at halfway. Because I am doing a bronco I am doing Vtail mixing using the built in mix but those don't appear to use Aux1 and instead mix REL and LEL.
Throttle Stick -100% > Thr -100%, Aux1 -100% < Good
Throttle Stick 0% > Thr 0%, Aux1 0% < Good
Throttle Stick 100% > Thr 100%, Aux1 0% < Bad
The really weird thing, I also tried just using the "Channel Assign" setup as well just to verify the motors gave equal thrust. It gives the same behavior
I had thought I had it setup right for differential in Fall when I maidened my bronco but once I throttled up it pulled HARD right and resulted in a crash. That is why I was checking the mixes and noticed that above 50% throttle it just does the wrong thing. It is getting to flying season and I wanted to be ready to give it another shot once my field is no longer flooded.