Are you going into PMIX2 to do your second mix?
Make sure in PMix 1 where you are negating channel 6 in the radio that you do not set this to ANY switch, just leave the switch to ON.
Then go into Pmix2 and do your second mix.
It sounds to me as though you do not have the first mix fully done, try flipping the channel 6 switch to see what this is doing to your servos direction, I think you'll find it affects the other mix.
Pmix One should be;
-100%
Master and Slave both channel 6
Switch ON (above this you will see another smaller "on")
Pmix Two should be;
+100%
Mast 4 and Slave 6
Switch (your choice, I picked DR switch)
Sincerely,Krysta
Product Development & Support Specialist
Futaba Service Center USA