User TheBusGuy on the Steam forums has written a "guide" to playing with two players on one PC. It requires the use of an Xbox 360 controller connected to the PC, but assuming the player has a controller it's a relatively easy process.
Meanwhile, fans anxious to test drive the zombies from the versus mode can get their fix--in the single-player game, anyway. Another Steam forum thread by user Scuzball explains how to switch over to the undead side and start a game against 4 AI survivors using just a few console commands.
Some players, like Steam user TIHan, are anxious to get started on custom mapmaking for the game. He discovered that Left 4 Dead already has rudimentary support for custom maps when he loaded up Dust, the timeless Counter-Strike map. The result was the video seen below: