Are you sure your network is properly configured for multicasting? My in-memory replication problems were due to a network that would pass a basic connectivity test. But multicast threads would not be renewed after 15 minutes.
What you need is some java code that will receive and display 100 multicast messages. At Oracle's metalink site you can download MultiCastTestReceive.java - which can be easily modified (sorry as I didn't write it, I'm not comfortable posting the code).
Start the app servers on both nodes. Run you receiver code, piped thru 'grep tcp'. You should messages from both nodes. Wait 20 minutes.
Run your multicast receiver | grep tcp again. You should still see messages from both nodes.
In my case, after 20 minutes, I would only see messages from the local node.
[Message sent by forum member 'updmst' (updmst)]
http://forums.java.net/jive/thread.jspa?messageID=348686