To do multiple clips of video on the same screen, add the clips to the timeline at the same time, then scale them to the desired size and reposition. Animate position with keyframes. That's the plain vanilla version.
If you want something more elaborate, you'll either need fine powers of description when writing, or a link to example video... and if it's long, a time really helps. Nobody wants to wade through a 20-minute clip for an effect that happens at 17:25 into the video.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA