hi, just noticed this discussion.
For twin engine planes, to get differential thrust on rudder you need to set two separate outputs to the two throttles. If the left throttle is on channel 5 and the right throttle is on channel 6 then you would set:
then also set the MIN/MAX values for those channels as @aamadeuss suggests
You can control how much differential thrust to apply with rudder using the RUDD_DT_GAIN parameter (it defaults to 10, meaning 10% differential thrust for full rudder input)