You will need the Directory Synchronization app in order to have your sites contain the same user, accounts, and security context. On top of this you will make site B a backup domain controller to site A. It will refer all directory changes to the master over your vpn when available and be 'read only' when not available. This 'read only' condition refers to the directory status...not shares. This means that if your VPN is down, users can authenticate just fine but they cannot change their passwords or add new users.
Once in this state, you can share things on either location to hosts on the domain at either site. Additionally, users can map network drives across the VPN but the speed of access is restricted to the ability of the pipe. For performance, caching of the authentication and user information is handled by the replication services of the directory. As for data replication, this is beyond the scope of the 'directory'.
Content filtering policies can be copied between sites for sure. If the content filter is set up in the EXACT same manner on both locations, this can be as easy as replicating the configuration structure in /etc/dansguardian-av. Since the usernames and accounts are exact between sites, a similar policy would work on either server.