Must admit I am a little puzzled? - hope i am reading the problem right.
Got the same mtroniks setup, wired as per the diagram in the main-sites technical stuff article, 'do they both go round mister', and works perfectly.
Also works if cheap turnigy v-tail mixer substituted for mtroniks mixer.
My setup needs slight throttle before steering works, whereas from the 1st post, it seems to read that rudder needs to be applied before throttle operates.
Maybe something to do with which/ order of channels it's plugged into, or position of y lead?
My radio gear is cheap, 2 channel, non computer type, so nothing to confuse the setup.
I also use separate receiver supply, so could there be interference from internal bec(s) of esc's?
Hope that helps, because few things are more frustrating than new kit not working as expected.