We’re not sure why you’d want it, but if you’ve been dying for a spit-screen solution to the PC version of Borderlands 2, then here it is!
Posted in Gearbox’s official forums, the guide lists a range of essential tools that you will need to get dual-instances of Borderlands 2 running in a single Steam account or on a single copy of the game. The walkthrough allows you to properly configure sound and controls for both screens. It specifically uses two PS3 controllers, but also mentions that you can use two Xbox 360 controllers.
Be warned, the walkthrough is pretty technical, and it’s also not officially endorsed by Gearbox, so tweak at your own risk.
Source: Gearbox forums