It seems like you could wire pin 20 (ground) and pin 3 (throttle signal) from each controller to the same throttle, assuming it’s a 3-wire (potentiometer or hall effect) throttle. It might be best to only connect pin 4 (5V supply) from one controller to the throttle. Keep in mind with this configuration the controllers are not isolated from each other as they will have a common ground.
.
