You need to remove the domainBackupMX=yes in order to trick the backup MX server that ALL domains are to be used as backup MX.
Yes you then un-check "Mark as Backup MX" setting, as this will be synced across from the main MX server, which will need to be marked as the main MX server, and not a backup MX.
Settings will autosync from the main server, however I think you will first need to do an ldiff export/import to get the two servers initially in sync.