There's a couple ways to install the servos and servo arms for dual elevators.
The optimum way is to mount the servos in the cabin, on a conventional servo tray, and hook up each pushrod on the same side of the servos.
Back in the tail, as in the U-Can-Do, the conventional method results in precisely what you've gotten, unequal travel between the halves.
The best cure for this is use an electronic Y-harness which electrically reverses one servo. Then both wil move the same..
The picture shows some of the possibilities..
The second version can use a conventional Y-harness.