Here is the solution I came up with...
After your movie is complete (with "to be continued..." and "part two" titles in the middle), quit iMovie. Open the project folder, and simply duplicate the project file, then rename both files ("My Movie Part I", "My Movie Part II") and double-click Part I. When iMovie loads, select second part of the movie in the timeline window, and drag it back to the shelf, then save and close the project. Repeat the operation with the second file, moving the first part to the shelf.
You will end up with two exportable project, sharing one media folder and identical styling. Be careful, if you edit something after the split, NOT to empty the iMovie Trash - that could delete media clips used by the "twin" project.
[Editor's note: In order to drag clips back to the shelf, you need to have the timeline viewed in "clip" mode, not "timeline" mode. This is a great little trick if you create longer iMovie projects ... wish I would have known about it about six months ago, when I ran into the same problem!]

