NRAD2000:
Dual elevator is done two ways on this plane: STOCK and MODIFIED.
The STOCK guys bolt elevator servos into the respective holes as the ARF is built, and the elevator servos can be Y-harnessed together, horn axle to the same side (forward or aft). When the LH servo goes clockwise, a topside horn will push up elevator.
When the RH servo goes clockwise, a bottom side horn will push up elevator. At or near center, its all good, but as you go to higher throws there is some inherent differential in the throws with this stock setup- consistent with what you are seeing. As you make elevator trim inputs, however, the two don't move together...quite a hassle IMHO.
The geometry is not quite exactly equal, but it will fly OK.... roll inputs from elevator differential aren't all that strong due to the short leverage (6") and area (30 sqin) compared to the ailerons (18" lever and 180sqin).
A good fix for this is to use a separate extension for each elevator servo and plug one into ELE of the DX7, the other into the Gear channel (5) or some other open channel. It sounds as if you've done this.... because you can make adjustments. If you have
the stock setup, and each ele servo on a separate channel, and the link that cmoulder sent confuses you, too.... follow me. Basically I just put the first ele servo into ele channel, second ele servo on ch5, set both ch5 atv's to 0% so that the gear switch does nothing, and slave it to the ele channel 100% using Mix 5 or 6. Mixes 5&6 bring the trims across.

MODIFIED:
Some of us (1/2?) like the symmetry of having back to back servos, so that at high throws, the horn/pushrod/horn geometry is identical. We have modified the kit a bit, covering up or never cutting out the factory hole, and putting a new servo hole in for one servo, directly opposite the other. I managed to get a pair of HS-225's back there and they have been working great for me for >100 flights. If you do this physical mod, the horns can both be topside, and the geometry is mirror image, so the throws are fundamentally identical (perfect if servos & all linkages were identical, you could say). All you do with this setup is the same as above, but reverse the servo on ch5 vs the ele channel. Now you can subtrim ch5 until the two line up at neutral ele. and use ele trim to move them together. Then you just tweak the endpoints (ATV) of elel to get the desired throws and play with linkages (horn length at the ele, and pushrod length) until the ch5 servo creates the same throws. OK? I can give you more, but first tell me which hardware setup you have.
MODIFIED: