I'm not sure what you're referring to with two non-interoperable message apps named Messenger? There was MSN Messenger/.NET Messenger/Windows Messenger/Windows

I'm not sure what you're referring to with two non-interoperable message apps named Messenger? There was MSN Messenger/.NET Messenger/Windows Messenger/Windows Live Messenger, but those were interoperable (they were the same thing, just updates). They also managed to wrangle compatibility with Yahoo Messenger (although by then no one I knew still used Yahoo), and apparently Facebook Messenger, and then did a crappy merger/migration to Skype. I'd go back to those days in a heartbeat, and I was there.