In FCP, click on the oval shaped button (the far right of the three) thats just above the viewer window and select Image + Wireframe mode. Then use the crop and arrow (scale) tools to resize the video windows however you want.
Take a look at Digital Heaven's "DH_Box" filter. It makes working with split screen easier. You can resize and reposition clips way easier than you can working with the motion controls in FCP. Do a google search for either DH Box or Digital Heaven to find their website.