The HomeAV standards do indeed allow you to have more than one set of powerline adapters in use on the same mains circuit at a time. They need to have different passwords and the performance is usually atrocious, but it will work! Obviously, you can have multiple powerline adapters in a set, but even then with HomeAV and HomeAV2 beyond 4 units, the performance falls through the floor!
While I've used Ghn (Devolo) powerline adapters I've never seen more than a single set in use at a time with these.