You should reconsider this approach. If you wanted two seperate database servers, then do seperate servers. But if you have a master/slave, then you should utlize MySQLs replication tools and techniques to get the most out of it. The DB is going to be better at this than anything you can do on the client side (php being the client of mysql).

Replication as a scale out method is meant for apps that rarely insert/update, but do lots of reads. More info here: MySQL :: MySQL 5.0 Reference Manual :: 15.2.3 Using Replication for Scale-Out

If you do more writes than reads, or have a mixed result, consider a mysql cluster as a scale out solution: MySQL :: MySQL Cluster