The files in the urbackup folder are part of urbackup itself and its management of the backups. No backups are held here (though their management items, e.g. backup_server.db, are. The backups are held in folders, one per client, a level higher, alongside the urbackup folder.
My recommendation (and my practice) is to install urbackup server on the client you want to hold the main server backup and then urbackup client on the main server.