This mod shows how to get split screen to work for Left 4 Dead. It has everything you need to get it to work. The instructions are below.
After the DLC/update, split screen stopped working! After a while of messing with it, I figured it out. So now split screen works again!
Posted by Croco15 on Oct 7th, 2009
I have figured out how to get split screen working after the DLC/update. I updated the summery, so you can take a look at that now. I doubt you care, but it took me three days of messing with it, to figure it out. I have also added a video, to make it easier to understand. Enjoy having split screen back!
Sorry for the horrible quality.