Having created obackup script in order to make reliable quick backups, i searched for a nice tool to handle two (or more) way sync scenarios in a reliable way.
Osync uses a master / slave sync schema. It can sync local or remote directories. By definition, master replica should always be a local directory on the system osync runs on.
Also, osync uses pidlocks to prevent multiple concurrent sync processes on/to the same master / slave replica. Be sure a sync process is finished before launching next one.
You may launch concurrent sync processes on the same system but only for different master replicas.