I configured two JBoss clusters in the same LAN: a production and a test environment.
I decided to configure every single cluster with a dedicate private LANs using a restricted netmask to isolate production and test connectivity, so I assigned
192.168.100.0/255.255.255.0 to test and
192.168.200.0/255.255.255.0 to production.
I configured Apache and mod_jk to loadbalance activities between cluster instances.

The page UsingMod_jk1.2WithJBoss (http://www.jboss.org/community/docs/DOC-12525) is a good tutorial to achieve this.

What problems should I expect?
JBoss uses UDP multicasting to replicate informations across cluster nodes: even if I isolate TCP traffic, JBoss will “ear” messages sent from other clusters and will log a lot of warnings like the following:

… WARN [NAKACK] […] discarded message from non-member ….

I had to change BOTH multicast ip address and port (attributes mcast_addr and mcast_port) in the following configuration files:

• ./deploy/jboss-web-cluster.sar/META-INF/jboss-service.xml

• ./deploy/jmx-console.war/WEB-INF/web.xml

• ./deploy/cluster-service.xml

• ./deploy/ejb3-clustered-sfsbcache-service.xml

• ./deploy/ejb3-entity-cache-service.xml

Good luck!