A poster on Gearbox's official forums has created a walkthrough (opens in new tab) explaining how to jury rig split-screen co-op into the PC version of Borderlands 2.
Member "Sycdan" lists the essential tools required for the MacGyver-ing of an additional instance of Borderlands 2 within a single Steam account with helpful step-by-step processes for setting up Steam, controllers, sound, and ensuring Borderlands 2's behavior doesn't send your rig in flames. You'll also get links to essential software needed for keeping the patchwork play purring. Just think: If everything works, you'll get a double dose of the most annoying gun ever (opens in new tab) . Yes .
Head on over to Gearbox's forums for the full guide (opens in new tab) .