I only know what I've read here and so am certainly no expert, but I was under the impression that the two didn't need to be on the same circuit, but to work best they must be on the same 110-v leg of the house circuitry, with most modern homes receiving 3-wire 220-v service, split into multiple 110-v legs for the various circuits.
It really comes down to the distance it has to travel.
Two different circuits, but on the same leg/phase of the 220, means they cross at the power panel. This may be no worse than if they're on the same breaker.
Now try going between legs/phases of the 220, and it needs to travel all the way to the transformer and back.
I've heard of crossovers being installed on power panels that allow bridging of what you want and blocking of what you don't want, but these wouldn't be "customer installs".
Romex was never designed for "high frequency", so it shows the talent of the manufactures that it works at all, and if you have "knob & tube", like the poster earlier with a house built in '35, chances of it working drop dramatically.
DSL works over twisted pair that was never intended for anything close to the frequencies used. Again how well it works depends on how far you are from the CO, remote terminal, VRAD, etc.