Are you talking about EJB/HTTP session replication or JMS clustering?
What is the motivation for having a server in a passive mode? That would be essentially degrading perfromance since 50% of the hardware will be left idle.
What kind of loadbalancer are you using? With mod_cluster you can configure a node "hot-standby" (by setting the load to 0).
I saw that by switching the IP address of the server in case of failure.
But as Radoslav already mentionend this is not the purpose of a cluster.
What you might do with WildFly is to implement a EJB NodeSelector and name the nodes Primary* and Backup* and as long you find a node named Primary* you select this one.
But it will make no sense to me, only make the environment more complex.