Picture in Picture Webcam Recording on a Mac?

If that was a live feed then perhaps they used software like BoinxTV or a hardware box to feed two singles into one.

If the video was a recording edited later, then all they had to do is record onto two cameras, import that video, and then use an editor to combine the two videos. iMovie can do that.

If you are a Vimeo user, why not just post the question to that account.