having one common Google account to host the calendar for both is a good workaround - interesting it doesn’t work.
I think you outlined a solution: create two calendars each and subscribe to each others calendar. While this will make you miss out on “collision alert”, it’s guaranteed to work (but possibly with a delay, see link 2)
- Using shared Google calendars on /e/ - where I suggested the same
- Show /e/-calender in Google-calendar - no changes reach Google - this is not about Google-to-Google share, but still helpful for knowing that there can be a delay when importing Google-external ics calendar subscriptions