Two - way MySQL database replication

MySQL database server has a master-slave replication feature, which makes it possible to have a complete copy of a master database on a slave server. When the master server fail, you have a copy of database available immediately, so you can switch your applications to the slave-server while bringing back up the master server. One drawback of this master-slave replication is that it is a one-way backup. The slave server is a complete copy of the master server. All changes that are done on the master server are applied to the slave server, but changes to the slave database server aren't applied to the master server.

I found these two articles are best for two way replication

http://www.neocodesoftware.com/replication/
http://onlamp.com/pub/a/onlamp/2006/04/20/advanced-mysql-replication.html