This is a current limitation of HornetQ http://hornetq.sourceforge.net/docs/hornetq-2.0.0.CR3/user-manual/en/html/ha.html#d0e9456
When a backup takes over, it is not currently possible to restart the live server without stopping the backup server first.
This is planned for HornetQ 2.1
This is actually pretty common behaviour. ActiveMQ for instance does the same.
However, as Jeff says we're going to add functionality in 2.1 to allow a new backup to be associated to a live while running.