Try this

Make a p-mix that has channel 2 as the master and any empty channel that your not using (like channel 5) as the slave.
Set up amounts to 100% both ways, and then move the right elevator servo (I assume you have the right side of the plane on your main channels) connection from channel 2 of your reciever to channel 5
Know try and see if they move the same.

One other thing, if the subtrim to one servo is set past zero (-47 for example) and the other is at zero you will have to even them out to let's say -27 and +27. Then you go back and set up your linkages to even everything back to normal.

The 9c manual states something on page 32 that I always wondered about, and is the reason i think that you and many others I know are having a problem with there twin elevator setups using the ailivator function or making up two mixes yourself.

They state "Ranges from 0% (no servo movement at all) to 140%. At a 100% setting, the throw of the servo is approximately 40° for channels 1-4 and approximately 55° for channels 5-8.•