I am by no means an expert but I have accomplished this by mixing the two channels using a free mixer, enabling the mix on the Aux 2 switch (page through the options on the mixer page to get to the switch enable/disable screen), then use Code 17 to disable the Aux 4 switch.

There is probably a better/easier way, hopefully one of the real experts will chime in. But this should get you going for now.

barry