Here is a tutorial with an easy process for manually creating a split screen during film editing.
Film image framing and aspect ratio can be a confusing element of the filmic process, especially during the shift to HD formats. Though it is not as common as other types of video effects, many users want the ability to have more than one video image present in the frame at any given time. This can indicate a number of different options such as picture-in-picture or artistic video collages, but the most common of these is the split screen. You will do the split screen effect during the film editing process, and can be very easy in non-linear video editing programs like Final Cut Pro. Here is a look at film editing for split screen and how you can do it easily during post-production.
Basic Split Screen
The basic principle to follow when film editing for split screen is that you are just allowing two different videos to stay on the screen at the same time, so you have to scale each so that they both fit in the available area and match each other. Start by bringing one of your clips into the Timeline in your program. Select the clip so that you can see it in the Viewer and make sure that the wireframe is available in the Canvas. Resize the image so it is 50% of its original size. Once you have done this go ahead and take that image and bring it all the way to the middle left or middle right, so that it hugs the edge of the screen.
Bring the other clip in and lay it on top of the first clip in the Timeline. Reside this second clip to 50% of its original size, and then hug it to the opposite side of the first clip. From here you will then want to adjust the sizes of both clips so that they look similar and are lined up along the middle as much as possible.
The reality is that film editing for split screen is rarely an exact science. The aspect ratio of different film clips is often different, their position is different, and you will often not take up the entire available screen when doing split screen unless you crop the images and blow them up. The reality is that you will usually have to do a lot of repositioning before you will actually be happy with your split screen effect, and you usually only want to try to do this during your film editing process if you have filmed both the clips you are using with the exact same processes. This means the same stock or HD settings, aspect ratio, bit rate, frame rate, etc. You can try to use opposing video types, but you are going to have to do a lot of image manipulation during post-production.