Assuming you are using JR equipment 8103, 9303, or 10X... you would use channels 3 and 7. Plug the right elevator into the ELEV (Channel 3). Plug the left elevator into Channel 7 or (SPOI). Use Mix 5 or 6 as they are "including" mixes...meaning the elevator trim will operate both channels (important). In the Mix setup....you mix ELEV to SPOI (Channel 7) and make sure the percentages in both directions are 100%. Make it "ON" instead of switch activated.

You will still be able to move the SPOI surface with the AUX 2 Channel. You can use another mix to "turn off" AUX2