Without seeing any other pictures/video, I would have to suggest that a combination of travel limiting and arm-to-linkage angle would be your best solution.
1. make the angle of arm-to-linkage as close to 90 degrees as possible on both servos. Seems like the "upper one" is no issue. Just change the "lower one" so the link and the arm at a 90 degrees to each other regardless of the angle of the arm to the servo body. Of curse this is at neutral servo and neutral elevator position.
2. adjust your servo travel until you reach equal travel on both elevator halves. Using a current meter in this step is vary valuable to reach the limits of elevator travel without binding the servos. A spike in current at the extremes of throw will indicate a binding problem.
Hope that helps
Rafael