Split from this thread.
I also have this problem. I added an additional exchange calendar ics file to the main exchange calendar in Apple Calendar. and it has merged the two. The pop up box was not a clear enough warning that this is what would happen and that it was irreversible.
I have tried removing the person from the shared permissions of the additional calendar but when I switched on the calendar again it still synced and displayed the additional calendar events within their own personal calendar.
I would be grateful for a solution to be able to un-merge the calendars.
Many thanks