I agree with you that if we want to achieve the highest possible flutter speed, each elevator should be balanced independently. Balancing the two elevators as a pair will work, but the flutter speed will be lower, due to compliance in the structure, which could allow anti-symmetric elevator flutter.
I suspect you have the new counterweight design, with the long axis of the counterweight running from left to right. My very slow build RV-8 had the older counterweights (pictures
here), with two weights on each side, with the long axis of the counterweights fore and aft. I've got electric trim, and I had enough weight in the counterweights to balance each elevator independently. So, one solution might be to use the older counterweights. I don't know which other parts have changed in the elevators, so I don't know if this will cause you any other problems. My plans are at the hangar, so I don't have the part number here.