Since the Adapter Sets are isolated from each other because they are in different classloaders, they can communicate with the same techniques two different processes could use to communicate.
But if you want them to see each other at Java level, you can leverage the "shared" folder. There you can put classes that will be inherited by the classloaders of both Adapter Sets.
So, for instance, the code in two Adapters belonging to different Adapter Sets can exchange messages by accessing a singleton of a class supplied in the Shared folder and use it as a bridge.
This feature is available if the Adapters in the two Adapter Sets configure the <classloader> property (see the previous post by Giuseppe) as "common" or "dedicated".
BTW, Giuseppe suggested the "log_enabled" setting instead. For this new purpose, you cannot leverage this option, but this just means that your Adapters will have to include their own log infrastructure (or lean on the JDK).