I have found a solution for syncing multiple office 365 calendars from different accounts and domains, by using extra calendars as an in-between and by using a free and open source plugin called CalDav Synchronizer (https://caldavsynchronizer.org/).
The way I solved it is in 3 steps:
1. I have a sync profile for 365 Calendar A with a Nextcloud calendar A which I host my self (but you could also use a Google Calendar A for this), only for events with category A or no category. (replicate sync profile) For every event synced this way, the category color in Outlook is assigned which corresponds to category A (make sure to set the correct color manually in Outlook)
2. Then I have another sync profile in the plugin for a different 365 Calendar B (within a different domain), which imports new and edited events from Nextcloud calendar A, but only for events with the before mentioned category A, and again, assign the category color of category A. (merge sync profile)
3. Repeat these steps for reverse sync, so use 365 Calender B, Nextcloud calendar B, and category B (with a different color) in step 1, and 365 Calendar A an Nextcloud calendar B in step 2.
The categories are used to prevent an infinite loop of syncing the same event back and forth.
Cons: This is a local solution, run as an Outlook Plugin on my laptop, so it only works if my computer is turned on and Outlook is running. Sometimes the sync pop-up hinders using outlook for a few seconds.
Pros: It can run automatically and instantly on every event change or event addition and/or only on a defined interval. If you want, you can set copied items as 'private', so the details are protected.
See also: https://sourceforge.net/p/outlookcaldavsynchronizer/wiki/Home/
Hope this helps.
