The Hypersonic database isn't a production ready database but it's shipped with the application server to let the users to start the server, just out of the box. Until it's replaced with a production ready database server.

To setup a JBM cluster, a production ready shared database must be used. This is highlighted on the community documentation but not on the EAP documentation :

"For a clustered installation it is mandatory that a shared database is available to all nodes in the cluster. The default JBoss AS uses HSQLDB for its database which is a local shared database. Therefore in order to use clustering you must replace this with a different shared database. If the database is not replaced then clustering will not work."

http://labs.jboss.com/file-access/default/members/jbossmessaging/freezone/docs/userguide-1.4.0.SP3/html_single/index.html#install.automated