Differential throttle mix for twin motors on a 9x radio (with ER9x firmware)

Testing for real a"diffrential throttle mix" using a ER9x firmware powered 9X Transmitter, using eePee simulation feature is a very good way to learn and understand how it works. (that's how I learned to do this mix) Here is the basics and link to the finished model plane ( )
The basics are:
The throttles outputs are Ch3 and 5.
CH5 is mixed 100% with CH3 as slave.
Then both have a mix with CH4 (rudder stick) to execute differential to one or the other, for that.. the mixes are opposite in value, one is positive the other negative.