Over and over, Valve underestimates the skill of the Source Engine modders. They release a demo for us, try their very best to keep us from diggin in deep into the engine to unlock features that Valve doesn't want us to see...yet. Once again, a modder has stuck his head into the inner workings of Left 4 Dead 2 and unlocked versus mode for us all to enjoy.
Dedicated modder
Saul created a plugin that allows anyone to play Versus Mode with anyone from anywhere. No extra downloading needed if you join a game in Versus Mode, just a simple command you type into console. If you want to host a versus game however, you will need to download a simple 80kb file and use a few commands to get you started. Keep reading for more screenshots and instructions on how to get this working.
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source : L4Dmods