When a child's life spans two homes, ordinary scheduling friction stops being ordinary. A missed handoff is not an inconvenience; it is a child waiting, and often an argument that has nothing to do with the calendar.
Shared facts, separate lives
The thing both households need is small and specific: the schedule, the medical facts, the school dates, who has what equipment this week. Almost everything else — finances, plans, private arrangements — should not be shared, and a system that makes sharing all-or-nothing pushes people back to text messages.
Write it down precisely because it is contested
In a low-conflict situation a shared record saves time. In a high-conflict one it does something more useful: it replaces recollection with a record. "We agreed Thursday" becomes checkable, which lowers the temperature considerably and protects the child from being the source of truth.