Sorry for late reply I had holiday
Normaly if you mix to chanels the slave is doing the same as the master.
If you press forward (+) the slave will do the same (+). If you press backwards (-) the slave will do backwards (-)
In our situation the pump is running only in one direction (+).
It´s not allowed to the pumb to run backwards.
So if you press (+) the slave must do (+),
if you press (-) the slave must do (+).
So on a diagram you will get an "V".
Who to configure with your sender, did you read the manual?