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Two-way sync of data between two cloud accounts means that changes (updates, renames, uploads, deletes) from the service will be automatically copied to other services and vice-versa: when you modify something on the second service the changes will
be automatically replicated to the first service.
One-way sync means only changes you make on the first service will be replicated to the second service. However, changes on the second service will not be replicated to the first service. One-way sync with option Archive files before they are changed or deleted by sync is basically a sync for backup.