I doubt it will even do that. It will probably just say "Upload Failed". Thats what has happened to me in the past when uploading videos longer than 10 minutes to YouTube in safari. Doubt iMovie handles it any differently.
+1. Split Part 1 and Part 2 at a logical point, upload both, create an annotation on the Part 1 video near the end that says "Click Here for part 2" and have it link to the video for Part 2.