[Stratoscope]

One way to think of it: When you take a picture with a camera, different objects in the photo may be in or out of focus, depending on how far away from the camera they are.

Closer objects may be sharp and distant objects blurry, or vice versa. Or somewhere in between.

Of course you can aim the camera one direction or another to choose its field of view - which objects appear in the frame and where - but that's completely separate from which of those objects in focus.

Focusing is one thing, aiming the camera is another.

And that doesn't change at all when you have two cameras. You can aim them both at the same thing, you can aim them off into the distance, but you still have to focus them both.