Sean: Your first two paragraphs are a given of course - I was jes' funnin' (and am not one to talk when it comes to freely dispensing 'authoritative advice' that amounts to little more than my take on common sense, bountious caveats included gratis :-). You may still well be correct in your basic presumption here, I just may not be able to take advantage of it.
As I say, the possibility of the second PLC tossing a spanner in the works does exist, but I could test this by plugging it in to same wall duplex as my regular one and trying this hook-up again, and then doing what you suggest in the last paragraph. The outlets in question are confirmed as being on separate breakers, but on the same side of the box.
I did notice something else today though - the two outlets are oriented 180deg. upside-down from one another, so that the polarized wide plug-blade slots are on the left with one duplex, on the right with the other. Since the adaptors I'm using are also polarized, and I am grounding them at the center screws in each case, this means that by necessity, both the 3-prong PLC wall plugs were plugged in oriented in opposite directions. I don't know how wiring behind the outlets looks, so I don't know for sure if this really resulted in the two PLC's being plugged in with reversed respective polarities, but the evidence is suggestive.
As I say, the possibility of the second PLC tossing a spanner in the works does exist, but I could test this by plugging it in to same wall duplex as my regular one and trying this hook-up again, and then doing what you suggest in the last paragraph. The outlets in question are confirmed as being on separate breakers, but on the same side of the box.
I did notice something else today though - the two outlets are oriented 180deg. upside-down from one another, so that the polarized wide plug-blade slots are on the left with one duplex, on the right with the other. Since the adaptors I'm using are also polarized, and I am grounding them at the center screws in each case, this means that by necessity, both the 3-prong PLC wall plugs were plugged in oriented in opposite directions. I don't know how wiring behind the outlets looks, so I don't know for sure if this really resulted in the two PLC's being plugged in with reversed respective polarities, but the evidence is suggestive.