Amku,
Your question really depends on a few issues. The first is if you use LDAP on your version 8 or not. If you do than I would upgrade the Control Center First then the Scanners. During the upgrade process the scanner that is not being processed will still pass email through. If you are using LDAP and you upgrade your scanners first they will spool your emails until they are added to a version 9 control center with the Directory Integration Settings filled in and enabled.
The Short Answer
Upgrade the Control Center
Upgrade one of the Scanners
Upgrade the other scanner.