two users writing to the same shared Google calendar - I don’t think this works with caldav.
What certainly works is subscribing to each others calendar - read-only by the counterparty. In the calendar view both agendas are visible, in a color that can be made the same. If you subscribe to be reminded on items in both this should work? agenda item collusions are not taken care of though
Haven’t looked at this for 1-2 years, thinks can change.