Final Cut - Stationary Splitscreen.

Drag two videos from the Browser into the Timeline ( into separate timelines, V1 and V2).
Click on the video in V2, this is in front.

In the Canvas window click and hold down this button:

Select Image and Wire Frame.

Now select the Crop tool from the Toolbox.

Hold your cursor near the left hand edge of the video in the Canvas Window until the Crop Tool icon appears. Now click and drag the edge of you video until it is in the middle of the screen.

If you Render and Play you should now have a split screen movie.

You can do this with as many layers as you like.