Here is an easy iMovie tutorial on how to combine two movie files.
Non-Linear Video Editing
iMovie is a non-linear video editing system, even though it is a scaled down version of this for easy consumer use. What video editing really means in a non-linear system is that video files are captured or imported into the program, that then references the base files from which they came. From here you will cut them together in a Timeline type form, and the final cut together video will then be exported as a single video. This means that you can import one video and cut it up, rearrange it, shorten it, or add to it in any way that you see fit. So how do you combine two movies using iMovie? It is actually simpler than it seems. Here is a quick tutorial on how you combine two movies in iMovie in a fast and simple way.
Combining the Clips
The first thing you have to do is identify the two videos that you want to combine using iMovie. Open up iMovie and begin a new project, and you should set the project settings for things like the aspect ratio according to the specifics of the files you want to combine using iMovie. Go ahead and go to File, Import, and Movie. This will open up a browser that will allow you to select the files you want to bring in. This will bring the clips into the Event window, where they may be in several frames depending on how long the clips themselves are.
The purpose now of your non-linear editing project, and in an effort to combine two movies using iMovie, is to select the clips and bring them into the Project window. iMovie allows you to select what section of the clip you want, if not the whole thing. Take each of the two clips and bring them into your Project window, arranging the order of them according to how you want. Now when you play through the Project window you will find that both clips play, depending on how much of them you dragged into the window. This is a similar process to doing it in a larger non-linear video editing program like Final Cut Pro, but just a simpler process for combining simple clips.
Now that both the clips are in the Project window and will play through as one, you must export it so that it will stand as a single clip, including both source clips. Go up to Share and you will have a variety of different options for iMovie export. You can select iTunes if you want to send the file over to iTunes, and if you want to burn a DVD using iDVD you can select that option. A basic export is going to be Export Movie, or Command and E on your keyboard. This is usually going to be the best option since it is the most basic way to create a clip with both. You can also choose to Export as QuickTime, which is just going to allow for a high quality QuickTime file.
Source: Author's own experience.