Don't view "picture in picture" as a special feature, because it's not. Every video editor that allows you to resize the video can do this. All you do is put the multiple video inputs into the timeline, resize the one you want to be picture in picture to be smaller and place it where you want it overlayed on the larger video. It's exactly as simple as grabbing a piece of clipart, resizing it and placing it on top of another image in an image editor. The only difference is the time componant.