It's been a while since I was last able to look at this, especially since I changed jobs in the meantime (and lost my backup mail server thanks to that job change).
rojozak: As far as I can remember, this step is unnecessary because the primary server is simply receiving mail from the backup server that go to local accounts. Essentially, I haven't had problems with the primary server rejecting mail from the backup server. Have you witnessed any problems without this change?
Some other information:
If you're running postgrey for greylisting, you should list your backup server in /etc/postgrey/whitelist_clients.local on the primary server for performance reasons. If you have greylisting running on the backup server, no need to have the email delayed again when it is relayed from the backup server to the primary server!