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.
This command will align the Scene’s Active Camera to the view,
And not the camera that is selected as the Active Object.
To make the selected camera the active camera:
In the 3D View select: View > Cameras > Set Active Object as Camera