You cannot back up VMs running on another node, even if they share the same storage on a SAN.
Application consistent backups require backup software to run locally on the node that hosts the VM.
Windows Server 2008 and R2 Specifics
You may only run one backup across all nodes of an entire cluster shared volume.
You can back up several VMs simultaneously; however, these must be running on the same node. In other words, do not overlap backups running on several nodes simultaneously. You need to schedule them to run without overlap.
Windows Server 2012 Specifics
Do not add local or other CSV paths to a backup task.
You must select a Hyper-V Backup task type in the task wizard. You cannot use Universal Backup task types to back up CSV VMs.