I endorse Tim's remarks entirely.
The mechanics of real-time synchronisation of data in a timely manner is VERY complex, and the recommendation Tim makes regarding a BIG but single DB server backed up in master->slave mode, seems to be the best solution that we have found as well.
If you have a LARGE organisation, consider multiple Moodle installs ( eg. one per school / department ), but using a single LDAP / AD based authentication system, which itself can easily have redundancy built in.
And use a media / document server(s) or a Hive system for file based media.
regards
Gary