By default, the view-ports Camera view in Blender displays the view from the camera that is set as the scene’s Active Camera.
If you split the view and in the new view select a different camera and set it as the Active Camera, the view in other view-ports set to camera view will update to display the new Active camera.
In short, by default, it seems you can’t assign different camera views to different view-ports..
But you can…
You just have to unlock the wanted view-port from the scene’s Active Camera by de-activating this button:
When the Use the scene’s Active Camera and layers in this view button is de-activated, your allowed to set an Active Camera for the specific view-port without changing the scene’s Active Camera setting, therefore not affecting other view-ports displaying the camera view.