The app supports a caldav endpoints so if you have a system which can sync this into some other provider it should already work. Currently the Calendar app itself does not have any way to actively reach out into other services. I guess the question here is which part in your setup is the single source of truth regarding the events. That one would be the server and the rest become clients. Wondering what the use-case is to have two such event stores itself. (Same answer really to the contacts topic)