You will need a reasonably powerful PC, two working XBOX Controllers and preferably two monitors / screens.
First connect your two controllers. Make sure one is player 1 and one is player 2. Locate your Borderlands2.exe, create a shortcut called "Player 1". Right click, properties, locate target. Here is how mine looks like: "D:Borderlands 2BinariesWin32Borderlands2.exe" -AlwaysFocus. Add the "AlwaysFocus" modifier. Create a second shortcut of Borderlands2.exe and name it "Player 2". Right click, properties, Target, "D:Borderlands 2BinariesWin32Borderlands2.exe" -AlwaysFocus -ControllerOffset=1 -SaveDataId=2. This will prevent the saving glitch and will make a separate save category for player 2 (SaveDateId handler). Start Borderlands 2 through PLAYER 1 using Controller #1. DO NOT USE THE MOUSE. Switch to windowed mode. If you are running Eyefinity or a multi-monitor set-up, move this instance of the game to that screen. I suggest a 1280x720 resolution to start the testing. Start another copy of Borderlands 2 using Player 2. Note: if you are running the game through Steam, please refer to this post. Thanks Elk_Cloner! Use Controller #2 to control THE SECOND instance of Borderlands 2. Make sure you are windowed and the controller works. Do not touch Controller 1, make sure you use Controller 2 to skip through the intros, etc. Download Gamers Window Relocator. The instance that is NOT on your primary monitor will reset back to your first monitor if you try full-screening it! Simply change the resolution to the resolution of your second screen and use the software to relocate your Borderlands 2 window appropriately and give it the "fullscreen" look. Switch network mode to LAN on both games, and join one game with the other. It should show up under "find games". Give your controller to your friend/girlfriend and enjoy. Source:
http://www.code4game.com/