multiple mysql instances in a cluster

Can anyone tell me what the best approach to building a cluster ha with multiple instances of MySQL Or for having multiple servers accessing the same MySQL instance (diff databases of course)?

I have two pairs of servers in an active-standby in a redhat cluster.

App1/2 have Service A running that requires MySQL Database A access
App3/4 have Service B running that requires MySQL Database B access

Option1:
One MySQL instance with both pairs having access to the MySQL service and databases A (app1/2) and database B (app3/4)

Option2:
MySQL service instance for App1/2 accessing database A and MySQL service instance for App3/4 accessing database B.

I'm running RHEL 6.4 on all the servers and the databases will be on a shared SAN.

Is there any performance differences with one option over the other?

Thanks,
Shane