Valve has released a new patch for Portal 2 which enables split-screen for two players co-op in the PC version.
You shouldn’t worry about dividing your screen’s resolution with your co-op buddy. The new patch adds support for Big Picture mode which allows you to play the game in the comfort of your living room.
To activate the split-screen the second player needs to press ‘X’ on their second controller inside the first co-op menu.
This feature of playing with your co-op buddy on the same screen was added to the PS3 and Xbox 360 versions of the game. PC like always got the feature late.
The update is live on Steam now and contains a couple of other bug fixes, like the controller glitches where the ‘quick ping’ button caused the player’s movement to stop and it was impossible to exit Robot Enrichment or Create Test Chambers menus using the controller alone.