There's an atom called "WindowCamera" which can be added to any scene and which has two controls: the basic "control" which represents the camera itself and can be positioned and pointed anywhere you like (you also need to check "Camera enabled" in its "Camera" tab!), and the 2nd control is called "DisplayControl" which represents the display showing the output from the camera, and again can be put anywhere you like and sized to any size you want.
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IIRC there should also be a couple of plugins that "emulate" that behavior i.e. on a screen of a mobile phone or tablet, and some you can add to a monitor atom itself for a second POV of your scene.