there is alot of different split screen that you can do but i guess you want to have two different clips playing at the same time, one on the left and one on the right. Put your two clips on the timeline, one on top of the other, double click on the first one so you can see it in the viewer, go to basic motion put the scale to 50 and the one you want on the right put 180 in the first box in centre and the one on the left put -180. Now you should have on clip on the righ and one clip on the left!
you want split screen that fills the whole screen. just put you two clips on the timeline one on top of the other, hilight the top one on the timeline and then in the canvas move it to the left or right. you can old shift while your moving it from left to right so it doesnt go up or down when you move it.
keep your clips at 100% and move one on the right and one on the left so you will see half of each clips on each side of the screen and its going to be filling the screen
there is alot of ways to had a border between them but the easiest one is to move you clips a little so you have a little bit of black in the middle. Or you can create a matte in photoshop and put it on top of your video in final cut
There is a Border filter that comes with FCP you can use.
Also, in the Canvas, turn on Image + Wireframe, then you can use the tools in the tool pallet to resize, rotate, crop, distort, etc, your footage with the mouse, fast and easy, right there in the Canvas. Way faster and easier than punching numbers into the fields in the Motion tab.
The easiest way to do this is with a TRAVEL MATTE. Make two mattes and use the CROP function in the MOTION tab to make them the size and placement you want. Them on your time line you will stack them (from bottom to top) Matte- Clip 1- Matte- Clip 2. Right click on Clip 1 and select COMPOSITE MODE-TRAVEL MATTE ALPHA. This will attach clip 1 to your matte. You can then move that clip within the matte for desired composition.
Rinse and Repeat.
This is the much easier than trying to crop two clips and keeping the sizees and composition under control.
Another question that the original poster might have is after you get your split screen the way you want it or resizing clips and keyframing them to move across the screen is there a way to save this as a favorite? Save the boxes or the boxes that make up the split screens so for future projects all you would have to do is drop new video in those empty boxes?
I see this effect a lot on CSI Miami and I wonder if they just have the effect saved and they just drop new footage in.