The clustering (i.e. load balancing of NMR traffic) of Fuse ESB is done using ActiveMQ Network of Brokers mechanism.
You can read a step-by-step guide on how to setup Fuse ESB cluster here:
fusesource.com/docs/esb/3.5/deploy_guide/DeployESBClusterConfig.html
and more information on how to establish a network of brokers on ActiveMQ (Fuse MB) here:
http://fusesource.com/docs/esb/4.4.1/amq_clustering/FuseMBClusterFaultTolerence.html
The general pattern suggested is to have a pair of master and slave on each host and interconnect the master of host A with the slave of host B, and vice versa.
However, if you interested just in ESB failover setup, it can be done using locking on the shared file system or database (jdbc):
http://fusesource.com/docs/esb/4.4.1/esb_runtime/ESBRuntimeFailoverFileLock.html