What you are talking about is PiP (Picture in Picture). There are many ways to accomplish this, but the easiest is to shoot, or choose images that are vertical, as video is horizontal. One would place, say the right-hand image on Video Track 1, occupying 1/2 of the video Frame. Then, they would place the left-hand side image on Video Track 2.
If the images need some tweaking, to get them to fill just their half of the Frame, the Crop Effect can be added, and the necessary "side" adjusted, as is necessary.
The same can be done with horizontal images, and the Crop Effect, plus probably the fixed Effect>Motion>Position.
I also show you a number of ways to do this -- step-by-step , with color illustrations -- in my latest book.
It's amazing all of the special effects you can create using more than one video track at once!
Thank you. I'll give it a try.
There is also another very recent THREAD on PiP, where the OP wanted to also animate the Scale of one of the PiP's, but keep it constrained. Steve and I both commented in it. Depending on what you want to do, it might prove useful too.
Good luck and happy editing,