Sorry for the probably silly question, but...
...how do I join them and then chapterize them in iMovie? I couldn't do this. (iLife '08).
If you're just going be watching it on computer, there's no need to join the files. Just use VLC to create a playlist with the two movie files and save the playlist to disk. Whenever you want to watch the movie, open the playlist file and it will run seamlessly.
That doesn't work if they want to burn a DVD (thus the "Help with iDVD" title).