I bought the game on Steam.  As soon as I sign in on Steam on one computer, Steam signs me out on the other computer.  If I duplicate the game in my steam folder and launch it, it quits since I already have another instance of the game running.

One of my motivations for doing this is to level up an alternate character.  Creating multiple copies of the program worked very well in WoW some years ago.  By duplicating the WoW program, I could launch many copies of the game and then have the alts gain XP in the same game as the main character.  

For older borderlands games, the advice is to use a third party program like game ranger for windows.  Is there a better way?