The problem would be multiple java processes not starting.
A cluster, pretty much by definition, is two or more JVMs working in unison to server up client requests. Requests are workload managed and spread out over the two or more JVMs, and if one JVM goes down, the other JVMs pick up.
The number of JVMs running will conform to the number of JVMs, or servers, in your cluster.
Also, just so you know, WebSphere will also probably be running a node service, so a machine will have all the JVMs, plus a node server. Maybe even a deployment manager JVM if that's running on the same machine as well.
Here's a link about WebSphere deployment and how Clusters and Application servers work together. Enjoy!
http://www.technicalfacilitation.com/get.php?link=deployment Sounds like you're cluster is working though. Good stuff!