Go to DISPLAYS MANAGER->DISPLAYS PROPERTIES and add your second monitor with the 'stretch main horizontally' option.
Install the latest catalyst control center and open up the advanced version. Thanks to overlordprime of the steam forums. Setting up your monitors with an ATI Card
If your monitors have different maximum resolutions you will probably want to select the 'Change Resolution' option from the left menu and adjust both resolutions to the highest possible setting of the lowest resolution monitor(1360x768 in my case). Your desktop should now span across both monitors. I choose Horizontal for the purposes of this guide. Select either Horizontal or Vertical Span according to your preference, either will work. Make sure SLI is disabled if you have the option, then select 'Set up Multiple Displays' from the left menu. Open the Nvidia control panel(Usually found in the context menu after right-clicking on the desktop). The secondary monitor should not be displaying anything(i.e.: Not enabled under Window's Display Properties - settings). Make sure both monitors are connected to your graphics card and are turned on. Setting up your monitors with an ATI card Setting up your monitors with an Nvidia card This will NOT work on Windows Vista as it does not support the 'Span' options. This guide is only for users running Windows XP.
JaytheHam's L4D Dual-Monitor Splitscreen guide
