This tutorial shows how to create a split screen effect with 2 videos, one on the left, one on the right, with each the full height of the screen. Of course, some of the video image will be lost because the window size for each video does not have the same proportions as the original video clip. You can zoom in on the video clip using Effect - Section and then position the section on the screen using the Effect - Size/Position. This can be extended to more than 2 videos.
The tutorial shows how to position by dragging the video clips around on the screen and resizing them using the handles of the clip. Alternatively, and for more precision, the numbers in the dialogue boxes can be used or modified.