How do you merge two separate projects in iMovie?

Export them both separately from one another, go to Share Menu > Export Movie...(choose the large size) and save them to the Desktop.

Now import the 2 separate .m4v files into a New Event folder. Create a new project, drag and drop those two clips into the Project Browser. Export that as the final finished version of your project.

That's an easy solution. However you did not consider the harmful effects of the additional compression. Please consider that the "raw" video is already compressed before it leaves the camera. Then again as it's exported from iMovie the first time. After merging the projects (with your technique) it get compressed a third time. And perhaps a fourth time if the movie is burned onto a DVD.

I'm very familiar with the loss of image and sound quality due to excessive compression, because I produced 43 separate makeup application tutorials for a manufacturer's website. Everything was fine until they decided that they also wanted me to combine all of them onto a DVD. My video was shot at high-def so that wasn't an issue. The issue was how to combine all 43 videos without a substancial loss of image quality?

I tried the obvious method (proposed by elikness) and found that my text graphics (still photos) suffered the worst - literally jagged black text that looked almost blurry. So I replaced all my graphic panels and burned the DVD. My client was satisfied. I was not. God only knows what the 90-minute (combined) DVD will look like after they've paid somebody to make 10,000 copies.

thank you - robert