All this seemed to do was stop the pan/crop from being resized from the centre, but the opposite corner to which I started it. I’m wanting to move the centre of the actually video itself.
Edit: I’ve added an image to try explain this better. I want to move the picture so that half of it is outside the red box, so that I can zoom in on the green box and then move it to the red box. This red box would take up half of the split screen, with a different video of similar size in the other half.