I think that DirecTV setups transmit power and control signal through the shield. A given wavelength carries two polarized signals and the control signal tells the LNB which polarization to watch for. (or maybe I've got it all wrong, but this is what I've gathered from setting up and troubleshooting my own)
You are right, though satellite transponder frequencies with different polarizations do not typically share the same wavelength. With alternating polarization they are offset by (presumably) half the frequency separation needed for identically polarized signals.
