[CentOS] Keeping data on 2 servers in sync !

I want to build 2 servers (both running samba) to provide file storage to 2
offices (approx 100 miles apart, linked via DSL) but all data writen to 1
server must also be saved to the other server. 
Both servers would also allow users to access the data via a VPN thus
allowing 1 office with a failed server to access the other server via the
vpn and still see the data from both offices.
I currently have 1 server working but we want to add the second office to
the system. (Currently 1 office has 10 users and the second office has 1
user connected via VPN ) but the second office will have 20 within 12 months
and the first will have 35 soon ))

Has anyone done anything like this ?

I am currently reading & unpicking http://www.linux-ha.org/ to see what that
gives me.

Any clues/comments very welcome, even if you think I am mad !

Thanks

Denis