Hi. In short, it depends on what you are trying to do. You cant add a volume from Controller A & B, to the same vFiler on ControllerA, if thats what you mean. Why would you wish to do this? If you are using separate IPSpaces etc, then generally the volumes are assigned to vFilers using that IPSpace on an individual controller.
Only volumes on the same physical controller answered already. And good point on ipspaces. Need to mark ipspaces for fail over. To use volumes on different nodes that concept is cluster-mode with Vservers which is a similar concept but across all nodes. But not available in 7-mode.