Split-Screen on PC

My girlfriend of two years recently expressed interest in playing Borderlands 2 with me, if I could set it up split-screen on my PC. I installed Sandboxie, and tried following this method but only one instance of the game will run at one time, the other says, “Installing Borderlands 2 - 0%” forever. The moment I take that instance of Steam online, it will run the program. This is true for the original copy of Steam and the sandboxed version. If anyone who’s a little more tech savvy could help me, it’d be much appreciated. I am on a Windows 10 PC and have no programs installed that could possibly interfere with Borderlands 2.