Both circuits are joined together on the common live and neutral busbars which are fed from your mains supply to the house. Powerline adapters use high radio frequencies which easily pass between the two circuits.
Both ring mains circuits start and end at the same point.
Provided the powerline adapters are not plugged in filtered extension sockets, sometimes called surge protected extensions, there should be no issues.
Ideally they should be plugged directly into the mains sockets, so pass through ones are usually best, if you need to plug anything else into that same socket.
