You are going at this wrong. You should have two "LIVE" control centers. You should have CC + Scanner + Scanner, or CC-Scanner + Scanner. Backup the CC and >IF AND WHEN< you loose the CC, use the backups to re-create.
SBG is not designed to have two replicated Control Centers. You are solving the reliability problem. echoing what TJ Davis said, If you build a CC-Scanner + CC-Scanner setup your users will need to check both CC's for spam settings, quarantine, etc. Not good.
You have two availability issues - mail transfer (the scanner function) and configuration/quarantine (the CC function). The most cirtical to solve is the scanner function, as you need continuious availability of the scanners. SBG can live for DAYS without a Control Center. So I recommend:
two scanners, load balanced via MX,
one CC - backup daily. Use the FTP option if you want the backups off the box.