Split-screen PC/Mac play is not supported by Valve via obvious in-game menus. The only way in which you can play co-op Portal 2 with a single Portal 2 purchase in a way that is obviously supported by the game manufacturer is by purchasing the PS3 copy:
Only PlayStation®3 split-screen co-op can be played using a single copy of the game. When you purchase a new copy of Portal 2 for PlayStation®3, you will receive an in-box code which provides your linked Steam account with access to your copy of Portal 2 on your computer. The game is still owned by only a single Steam account--another copy will be required to play online co-op with a friend.
It has been noted in the other answers that the act of single-system split-screen is possible. It required modifying configuration files and a workaround to force the game to move a controller to the second player slot.
Valve has neither stated that this method is supported or banned. Steam Support indicates it's not supported, but also openly admits it's an option that currently exists.
Thank you for contacting Steam Support.
Split-screen mode is available in the Xbox 360 and Playstation 3 versions of this game only.
However, split-screen options may be available using the methods found here:
Note: These methods are not supported, as split-screen is not an intended feature of the PC/Mac version of Portal 2.
If you have any further questions, please let us know - we will be happy to assist you.