Hi
Sorry to nit pick but when you start talking about dual rx you are into a whole different ball game to just comparing PPM versus PCM rx. Such a system would work just as well if not better with PCM.

Plus Unless the two RX where operating on different frequencies then the ambient signal to noise ratio would bomb both rx out at the same time. ( regardless off antenna orientation)

But interesting all the same.

The true answer is spread spectrum as it would jump to find an less crampy frequency.

Mike