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I think alison3492 is not understanding that this thread is about over the air (recieved thru an antenna) broadcast TV of decades before HD was even being developed.
I think the main reason that the carriers were in the same channel bandwidth was a consumer simplicity requirement. Many people did not buy radios until the TRF sets were gone or equipped with simplified one knob tuners. This was because the average consumer could not wrap their head around using the 2-4 tuning knobs of the average TRF set to tune a station. I suspect the engineers looked at that (then recent event) and thought 'if we tune the sound and video separately in different bands most folks will not be able figure out how to tune their new EXPENSIVE set and will get mad, bad mouth the technology, and this whole thing will be a flop'....As an example of this in action just look at how many consumers called the service shops to fix their color set due to 'no color' symptoms when all that was wrong was that they were too dumb to adjust their sets fine tuning knob....Which caused AFT (Automatic Fine Tuning) to be invented and take over.
I suspect most separate carrier systems (which were mechanical and experimental electronic mostly) were done that way so that they could take 2 out of the box AM audio transmitters, make a test station, and only have to creatively focus on the image systems....Having to experiment with RF design would only have distracted from electronic image system development that was sorely needed in the 20's and 30's...
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