how to share sessions between independent tomcat instances

I have several tomcat instances running in physically independent machines.

I want to configure the tomcat to share sessions between this instances.

I have tried to configure org.apache.catalina.session.PersistentManager from http://tomcat.apache.org/tomcat-6.0-doc/config/manager.html. But I only see the session file when I shutdown the tomcat instances and I don't know if the instances are sharing this session. I think not. Because It doesn't make sense if tomcat writes down the session only on shutdown.

The other thing that I found is the cluster-howto but I can't do that couse the machines can't see eachother. They only shares a storage path to use.

Other thing that I think I can do is to implement a manager but it seems a litle bit tricky.

I have to add that I am using tomcat for deploying grails war files and I am using the grails session. I thing it has something to do with Spring

So, the question is: What is the best thing you think I can do to accomplish more effectibly the task? Or maybe I am missing something? Can you give me any pointer?