what you proposing is equivalent to increasing the frequency of the channel. If both do the same "2-nd" order frequency - we're back to the same "1-frequency" channel problem. If different - that's outside of the "1-frequency" channel problem.
Each radio would have a unique 2-nd order frequency for it's own filtering use, as a "tag" of it's signal. The primary frequency is known and used for the transmition, and the 2-nd order one is piggybacked on it. I'm not saying there are 2 transmiting frequencies.
> I'm not saying there are 2 transmiting frequencies.
200 years ago, people might have believed you. Then Fourier showed that your statements contradict each other. He said a lot of other interesting things - Bracewell's book is a good introduction.
