A common question we get when multiplatform releases include the PC around Co-Optimus HQ is - "Does the PC version support split-screen too?"
Sadly the all too common answer is that the PC version doesn't support co-op, even though the console version does. Sure, there are exceptions to the rule, but 9 times out of 10 PC users are out of luck. The nice thing about a computer is that they are easily modified - and several crafty users have figured work arounds for games - including the recently released Portal 2.
To enable split-screen you're going to need at least one supported controller along with your keyboard and mouse set up. Other than that its simple console commands in the game and configuration changes to get your couch co-op gaming goodness. Extending the hack further, users with two monitors can tweak the game to play split screen with each player on their own monitor! That means you won't be constrained by a small view on a 23" screen!
Full details can be found in this forum post on the Steam Forums on how to enable split-screen play in Portal 2. Further details on enabling the multi-monitor hack can be found here.