actually i have implemented a portlet which runs on Websphere Apllication server. There will be two WASs and the requests will be given to them by IBM Edge Servers. I dont know the details about the edge servers, but i can learn if you wish.
I'm not that familiar with WAS. I believe, if you're using session replication, that the session events would be fired on all machines in the cluster (I'm not certain of this though).
If this is not the case, you could, if you are using a shared database, write your session counting data to it from both apps. You might also want to look into Java Messaging Service (JMS) and/or Java's Remote Method Invocation (RMI).
My own solution to this was to build a split-screen webpage that shows both servers, each in their own frame. Since I'm the only one who uses it, this cheap, 5 minute hack is more than sufficient. Your needs may vary.