[Premiere Pro CS4] How to do a PIP (picture-in-picture) in background so it doesn't block the video in front?

I'm doing a project in which I have 2 separate videos, the main and a background. I need to find a way to put the small background video in the main video's background, but can't figure out a way to do that without blocking the main video or having to use keyframes to move it. Any suggestions?