Google Apps as BackupMx

I am using Google Apps as a backup mx to the domains I host. All of my DNS records are now setup and work properly (had to fool Google to get this to work correctly). What I would like to do is setup dual deployment for mail not only go to my domains but the backups as well. There is a way to do this according to Google but the instructions are for Qmail not Postfix and I am not entirely sure how to do this from the GUI. Any suggestions?