I'm presuming you're talking about 2 powerline adapters, rather than some Frankenstein electrical adapter. My reply here is based on the former, the latter sounds more fraught.
You could certainly use a length of ethernet cable between two powerlines on two separate mains supplies and that would work perfectly well.
However, I'd be very surprised if they are actually separate supplies. Are they both ultimately connected to the same consumer unit? It might be that the round trip distance is just too long for a reliable connection. Have you tried connecting one in the separate supply and connecting to it?
The worst that could happen is that the ethernet traffic would interfere with the powerline traffic (if in they are on the same circuit) and cause network collisions, which would result in poor network performance.
There is no danger in stringing a piece of ethernet between them and seeing what happens. At worst, it won't work.
The best strategy for testing this would be to pair two separate sets of adapters. So A1,A2,B1,B2. plug A1 into router and A2 near the other circuit. Plug something into it at this point to make sure it works. (a laptop etc)
You could also try plugging A2 directly into the separate circult at this point and see if it works.
If it doesn't, get an ethernet cable from A2 into B1 then plug B2 wherever you need it and give it a go!
(I've no idea if that makes any sense at all :P )