I thought this was a new team calendar that was version controlled when I first read the headline. And now I want that.
If I want to share a calendar with my spouse, do I need to get two subscriptions? No. You can just create a normal subscription for yourself, and an "Associate" subscription for your spouse. Then share the calendar with the second account, and that's it. However, Associate accounts can't create their own calendars. So in order to have a personal calendar (that you can't see), your spouse will have to get a normal subscription too.
Then I thought about how meeting requests are basically PRs. How how easy it could be to create shared calendars using submodules and such.