replicating data over 2 servers

[nbari at unixmexico.com]

hi all, i have to servers (A, B) with round robin DNS, users enter to
server A and server B and they write to a txt file, how can i sync the
file from both server so it can be always the same? for example if a user
enter to server A and modify some data, replicate that modification to
server B so when user enter to server B he can see his changes, and the
same if user modify a file on server B, replicate that file to server A,
any idea?