Universal Messaging Realms may be set up to share a single virtual or physical disk, in Shared Storage configurations. While not technically a Cluster, Shared Storage configurations do provide a basic mechanism for rapid failover between Realms:

Note that simple Shared Storage configurations do not provide any form of load balancing or scalability, and rely on administrators to perform manual or scripted failover processes. We therefore strongly recommend the use of native Universal Messaging Clusters over Shared Storage configurations.