Multiple Phantom Cameras
When multiple PCams are in a given scene, the Priority property is what determines which one controls the Camera node. The one with the highest value becomes the active and, therefore, controlling one.
Switching a PCam's priority can be done either from the inspector within the editor on the individual PCam node, or via code - see the Priority page for more.
Changing the active PCam, when there are multiple in a scene, is also what triggers a tween, or interpolation, between different PCams - see the Tween page for more.
Visual Example
PlayerPCam has the highest Priority at 10
NodePCam now has the highest Priority at 20