Hi, Coleyb,
You cannot have two projects open on your screen at the same time. You could with the old iMovie 6, but not with subsequent iMovie versions.
However, you can close Project #1 and it will save to your projects browser (the screen where your projects are displayed as icons). You can then open or create another project, say Project #2, and edit the new project Then you can close Project #2. It will save in your projects browser like Project #1 did. Then you can go back to Project #1 in the projects browser to continue editing it. You can alternate between the two projects this way. It's just that you cannot have them open on your screen at the same time.
After you close any project in iMovie it will remain intact in the projects browser, with all your edits preserved, where you can later go back to it and continue editing. Even if you share out (export) the project to a movie file, or to email, or to a video website, you can always go back to the project in iMovie and share it out again and again, and re-edit it again and again.
-- Rich