Maybe in a block of flats or perhaps a semi-detached or terrace then interference could bleed into your neighbours circuit, but presumably if there's a sufficient enough gap between the mains power inputs, any potential interference caused in a neighbour's property should be negligible enough for it not to matter. I'm only guessing of course, with no real world data I can't do anything but guess. Having said that I just can't believe they're powerful enough to cause definitive interference outside the local circuit (again if it's not close to another one).
