As some existing threads suggest (eg, one, two, three), current Dialog Box does not provide an API to send regularly messages from the host page (eg, task p
I had similar issues communicating between apps. In my testing, Excel Online storageEvent works because the communication is between browser tabs of the same "instance". With Excel for Windows, 2 add-ins are in separate browser "instances"; which don't communicate storageEvents. Worse yet was Office for Mac (WebKit); which did not communicate storageEvents and also could not read storage changes from the other app unless they were restarted -- I believe this is because the storage values were cached for the instance (not written to disk and not visible to the other instance/app).
So, I believe Excel Online could use StorageEvents, Excel for Windows would require polling (e.g. check every second), and Excel for Mac may require a server-based solution, such as WebSockets.