Cross-harness agents messaging?

Within the same harness, agents get inter-agent messaging by default. But what happens when you want two agents on two different harnesses (and hosts) to communicate? When on the same host, I tried having them both leave messages to each other by writing to a file and tailing it to grab the other's response. Quickly, it grew to hundreds and even thousands of messages, and other agents/sessions started picking on it too, joining the party (e.g. maintenance session within the same project). How do you currently solve this? What are you using? I noticed Bolter (connect[.]bolter[.]chat) solving this issue, supposedly, and was wondering if anyone has good experience with it and can recommend. EDIT: forgot to mention that, ideally, I'd like to be able to monitor their communication.