Two geologists work opposite sides of the same ridge and neither can see the other's stations. At camp somebody merges files by hand at eleven at night and, about once a season, quietly overwrites a day of somebody else's work.
This makes that impossible. Every record carries the device that made it and the moment it was made, so merging is a union, never a replacement. Nothing can be lost, whatever order people sync in — and it also tells you where everyone was when they were last seen.
"Last seen" is the newest record that person has synced to you — not live tracking. If someone has been quiet for hours and you expected otherwise, that is worth acting on.
Export a bundle and give it to your teammate however you can: AirDrop, Nearby Share, Bluetooth file transfer, a cable, or a memory card. They import it, and both phones end up holding everything. Do it both ways and you are fully in step.
On a bad connection a bundle can be pasted into a message. Small payloads only.
Why a union and not a sync. Records are never edited in place and never deleted by a merge, so two phones that have both changed cannot conflict — the result is the same no matter who syncs first, or how many times. That property is what makes it safe to hand a bundle around a camp at midnight.