I'm not so sure a second switch is actually required, except in special circumstances....

I believe a second "method" of killing the engine is required, but not necessarily a switch.

Certain contests or field may require a "switch" in addition to the normal on/off switch. However, generally you are good if you can shut the engine off using carb cutoff (I use a setting on the transmitter, coupled with a switch setting on the Tx to engage that feature (much in the same way you'd use flaps), closing the carb, thus killing the engine).

On one of my gassers, I have an optical kill switch between the ignition battery and the ignition. The kill switch operates using a switch on the transmitter, and works to cut off the ingition voltage completely. The optical switch is made by Electro something...I'll look at it shortly and post where....

To answer, the only servo used in my scenario is the throttle servo. The optical switch needs no servo, and plugs directly into the receiver.

Hope this helps...

Sean