Syncing

This looks easy enough. From the iPad User Guide.

“Share an iCloud calendar. Tap Calendars, then tap next to the iCloud calendar you want to share. Tap Add Person, then enter a name, or tap to browse your Contacts. Those you invite receive an invitation to join the calendar.”

Excerpt From

iPad User Guide for iOS 12

Apple Inc.

https://books.apple.com/us/book/ipad-user-guide-for-ios-12-3/id1420662722

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This is done on the Calendar app. I had to crop the photos to be able to post them.

family sharing is good for shared calendar

https://www.apple.com/family-sharing/

other than that you can just go into settings->passwords & accounts-> and go nuts with the Add account

https://www.imore.com/how-set-mail-contacts-and-calendars-iphone-and-ipad

more

https://www.google.com/search?q=iphone+add+mail+account+and+calendar&oq=iphone+add+mail+account+and+calendar&aqs=chrome..69i57.11758j0j7&sourceid=chrome&ie=UTF-8

Assuming you both have your own Apple ID, you can share your calendar: https://support.apple.com/kb/PH2690?locale=en_US

Or, you can use family sharing as well: Family Sharing - Apple