What you want to do is to 2 long straight bamboo skeewers or something similar and either tape them to the elevators in the exact same spot or glue them to clothes pins.

Then you can attach them to the elevator and watch to make sure the surfaces travel evenly throughout the range of travel.  I would bet they do not right now.

You want to get your subtrims as close to 0 for each servo as possible.  Then adjust your linkage to get the surfaces equal. Remember that the distance between the control horn and the surface should be identical on both surfaces as well. Preferably the distance from the center of the surface to the center of the control horn connection should be equal to the length of your servo arm.

To get it right takes some time and patience, you will be surprised how much of a difference 1 turn on the ball link will make in the overall travel and how well it matches the other surface.

It can be a real pain to get it right, but worth it all the same