I have to concur with plasticman,

I just took a 5 channel RX plugged in a servo to Ch4 and then another on Ch2 and then assigned Ch2 to RU .

Indeed the channels do NOT operate independently of each other, Centering, Reversing etc affect both servos. Dosent' matter which surface channel you make the changes from, both change.

Did you try this as well?

Mike R