I have two iPhones running iOS 12.1.2, each with different Apple IDs. One phone (an iPhone 5S) has complete data for tiny calendar but incomplete contacts. The iPhone 5SE has complete contacts but incomplete calendar data. I would like them to sync. Any ideas?
If tiny calendar uses the built in calendar accounts like (some?) other third party calendar apps do, you can open Calendars.app and tap "Calendars" at the bottom, and then the "i" in a circle next to a calendar. From there you can share iCloud calendars that you have created.
I'm not sure about contacts. You can share contacts one at a time from the contact, but that doesn't keep them in sync. And I don't see a way to do it more than one at a time.