Try it this way.
1) hook first motor in throttle slot. (master)
2) put second motor in open channel (slave)
3) MIX 1 throttle -open channel leave on at all times (no switch) and use + or -100%
Check you motors that it operates.
The second mix is where you went wrong. You want rudder to open channel.(slave)
4) rudder - open channel - first make sure you put it on a switch so you can shut it off if needed.
5) set the % to about 15% for first shot. The object of the game is to get the right engine to reduce speed when right rudder is added. This is determined by either using the +15% or -15%. Once the proper sign is determine, set the opposite sign when you move the stick to the left.
Remember you reduce rpm's when right rudder is added for the slave motor.
The throttle can never be altered, the slave can.
If you go to the experimental section of this forum., look for Virtual Rudder Control for two motor plane. In fact, I'll do a video to explain it better.
I like to design and fly unique planes.