Hi Mugsman3,
I can fully understand the situation: user 1 create a secondary calendar, and then share this calendar with user 2. But user 2 cannot open this shared calendar. I have made some tests on my side, and I find the method to open this secondary calendar.
1. Let the Mac user click File -> Open -> Other User’s Folder.
2. Click Find User.
3. Type the name of the person who granted you access, click Find, click the person's name, and then click OK.
4. On the Type menu, click Calendar, and then click OK.
After that, the shared calendar appears in the navigation pane under SharedCalendars. We can see the secondary calendar.
If anything is not very clear, please feel free to let us know.
Thanks,
Young Yang