The Audio panel Transmit Knob Selector is not actually setting the Simulator Data as to which Com is the Active Com.   The Listen Buttons do set the sim Data.. And thus if you turn on both then turn off one the sim will flip the transmit under the hood if the one you turned off was the previous Transmit...   Ideally the Knob should be changing The Sim Variables for "COM TRANSMIT:1" and "COM TRANSMIT:2" in addition to setting the LVAR for it's value.  Which does work to change