RE: 4 elevator set up w 12x??
OK, I tried first assuming you have 4 aileron servos as well. So I set up under system mode, device select (inhibit flap, aux 5, aux 4, Flap.), choose 4-aile under wing type. Mate Aux 4 to elevator. I next tried to simply choose elevator to flap mix under the function list which works fine along with one p-mix, but the elevator trim did not transfer and you need to elevator trim to track on both servos on each side of the elevator.
So back to two P-mixes. I would not use multi-point. Just straight P Mix 1 for one side (elev+ to aux 1 and P Mix 2 for elev+ to aux 5) make sure you select elev+ rather than elev so it includes trim. You can adjust each direction + or - 100 percent or any appropriate percent to get your throws to match. Like any other type of multi-servo setup, try to match geometry to the maximum amount rather than relying on the P mix to mix out throw differences.
Double check everything under servo monitor to make sure all is flowing correctly.