What have you made so far?

Do you have any specific questions?
Problems with certain parts of the game/code?
For example, movement, collision, boundaries.


For a split screen you can basically do all the updating to one
DisplayObjectContainer(MovieClip) but don't add it to the stage.
I'll call this movieclip "content"

Next you can create a BitmapData (size of the screen) which is added to the stage via a bitmap container.
Code:
var canvasData:BitmapData = new BitmapData( screenWidth, screenHeight );
var canvas:Bitmap = new Bitmap( canvasData );
stage.addChild(canvas);
At every update you can draw certain aspects of content to the half of the canvas, which will fully represent the game resolution.

Code:
//matrix defines position to draw content onto canvasData
//bounds is a rect that should define the boundaries that each player sees
canvasData.draw(content, p1Matrix, null, null, p1Bounds);
canvasData.draw(content, p2Matrix, null, null, p2Bounds);
This code will not work, as I did not define many of these variables.
It's just an idea.
This is one way for split screen. I'm sure there are many others.