Edwards newsletters are wonderful! You will learn a lot from them. But quick and dirty:
You want to use cropping and track motion.
As an example, place car A on track 1 and place car B on track 2. Line the clips up so they are at the same entry point into a corner (or whatever you are comparing)under the curser on the time line. You can then use track motion (icon in the track header) to reduce the size of each track so that both will fit on the same screen. However they will now be right on top of each other and you will only be able to see the track 1 car. You then take and move the track 2 picture to the left (again with track motion) and will see it emerge out from under the track 1 picture. Move the track 2 to the left side of the screen and likewise move the track 1 to the right side of the screen. You now have basically a PiP with one 0n the left and one on the right. Hope this helps. And do read the newsletter.