confused: Dear frinds I need help with 9zap


I am using two elevator servo, with my zap in a edge 540 with a R 149 dp the instalation is as follows :the one servo is plugged into channel 2 and the second servo to channel 7 ,I am using programmable mix 1 to slave channel 7 to channel 2 and trim field on. So far so good , but I have different speed in each servo when I am adding full stick .
By the way the expo is setting the same (0) for both elevator and av2 . and also
the servos are identical (robbe digital st 250 )

Does any of you knows how I will fix this problem .????