You're required to broadcast on all frequencies when sectors/positions are combined. I wouldn't bother switching a pilot to a different frequency that I was also using unless an operational advantage can be obtained. For example, some frequencies have the ability to be cross coupled so if I have 3 frequencies but only two can be cross coupled, I might put them on a different one to avoid them stepping on each other. Also, if you're really combined up like on a midnight shift, then the quality of some frequencies deteriorate even before they leave your new combined sector.
Yes, the Center controller's can work multiple frequencies, but "cross-coupling" the frequency allows an aircraft on one frequency to hear aircraft that are on a different frequency. This is not the case with the old system where the pilot could not hear the aircraft on the different frequencies the controller was working.
It seems like you and spades are using two different definitions of "cross-coupling". I believe spades is referring to only having the controller transmitting on the multiple frequencies he is using, so if aircraft A is working on freq A, and aircraft B is working the same controller on freq B, then the controller's instructions to "A" will go out on both frequencies, and B will hear the controller, but can not hear replies from "A".
You seem to be describing what is called in radio hardware terminology, a "repeater" function, in the sense that when aircraft A transmits, and that audio is received at ATC, thenin addition to presenting that audio to the controller's ears, the ATC radio hardware also retransmits it out onto frequency B.
I'm not meaning to contradict you - I admit I don't know - but do you have any references explaining this technical detail? Whether ATC "cross-coupling" is just when ATC transmits on multiple frequencies being worked by one controller (spades' usage), or whether it includes the repeating of received aircraft audio transmitted back out onto a second frequency being worked at a given position (JetScan's usage)? I'd like to read up on it to educate myself.
Does anyone else have any details to add here?