Here's what I did to play with two local players (One on each monitor) and two joining over the net.
1. I set my two monitors to 1024x768 (Just to test, will try larger resolutions later) in a horizontal span
2. I altered the video.txt and videodefaults.txt in
steamapps\common\left 4 dead\left4dead\cfg
to say this instead of the other settings
I then locked those two files.
This makes a windowless border window that spans over both screens. The split goes straight down the middle.
3. I followed the initial guide to get the controller working.
(I had to unplug and replug the x360 controller several times to get it working, you have to do it real godamn fast)
4. I started a game using the ss_map command in the console. (Just choose the first map in a movie)
5. This game is a locally hosted game that people in your friendslist can join.
I played with me and my friend on the same computer, and two other joining through the friendslist.