How do I position each movie in split screen in iMovie?

iMovie has a slick feature to make it easy to do a split screen, but when you add the 2 movies and then do the split, it centers each movie. I want to create a movie where the camera stays in one place, and I record 2 separate movies with it, with me being on one side of the room in one movie and the other side of the room in the other movie, so that it appears that I'm 2 people in one room.

So, when the 2 movies are put together, each split screen continues to show either the far left or the far right of the video rather than being centered on each video. In other words, how to I define what area of the video is being displayed on each side of the split?

Thanks!