For a free solution, consider Quartz Composer. If you have Tiger, install the Developer Tools from the second disc. In the /Developer folder (I think), you'll find Quartz Composer. Just drag two movie files from the Finder to a blank project. If you take some time to learn how to use it, you could get some good split screen effects. You can then just drag the .qtz file from the Finder into iMovie and it will import, ready to use.