Put one video in the main video track, and one in a superimpose layer.
Right click on the video in the supimpose layer and select Video Option > Motion to bring up the following screen:
First select the Start point in the top right window. Now type the percentage you want the video resized to in the zoom box. (I would choose 49% here). Then click on the Start position in the top right window and move the video to where you want it located. Click the end point and again zoom to 49%. Move it to the same location as your start point. Set the fill colour to black (click on the area underneath fill color, then put zeros in Red, Green and Blue.)
We now need to set the transparency colour so that we can see the clips underneath. So right click on your clip again and select Video Options > Transparency. Now set the Key type to RGB difference and the colour to black as before. Click OK. It would be better to set the colour to green and use a green screen key, but I can't remember how to do this off hand,
Repeat for the second clip the seperate video track.