Why don't you join two instances of Jboss into a cluster, so that the session state is shared across the two instances. Put the two machines behind an apache load balancer using mod_jk.
Then, when it's time to upgrade, you can take one server out of the balancer mix, upgrade the codeline, and then put it back in the mix. Then do the same with the other. IF done correctly, nobody should lose their sessions.
The current setup is sticky load balancing. There are 2 application servers but isolated from each other.
Anyway, thanks for your idea