The only way to have a single Windows device in Storage Manager represent a cluster is if the cluster in active/passive. In this mode the Agent service will only be running on the active node, and the virtual IP will be used to identify this in Storage Manager. If there is a failover in the cluster the agent service on the newly active node starts up, and data collection continues.
In the case of Active/Active clusters the agent can be installed on each of the servers in the cluster, but then each server will be seen as a separate Windows device in Storage Manager, not as part of a cluster. This may result in some duplication of volumes that may be shared in the cluster.