Simple hack I just learned:
1. Use any broadcasting software (OBS, Streamlabs)
2. Window capture the video you want.
3. Resize the video so that only the part you want fills the screen, along with open space on one side or the other (make sure the aspect ratio of your screen and your window are the same).
4. Record the video from the broadcasting software.
5. Repeat step 2.
6. Repeat step 3, leaving space on the same side as the first video.
7. Repeat step 4.
8. Overlay one over the other so that the free space of the first is take up by the focus of the second.