I am not sure I understand your question. But you have a 8 channel RX and a 8 channel TX. If you used a Aux channel for the second elevator servo, then that still leaves three other AUX channels for other things on your TX and RX for things like a kill switch or a choke or something.
You can dial in the servo end points to match up with the choke OK. So you could use one of the  TX switches coupled to a AUX channel to work for that.  If you have an optical kill switch you could control it another AUX channel too. A mechanical on/off switch gets to be more tricky though. But you could use the choke servo to run both. You can use a spring and a small diameter wheel collar on the pushrod and a three position switch on the TX to control it all. The wheel collar pushes the switch on or on inside of the plane. The spring allows a little give for the choke servo  to exceed its travel some.