Two Video cards

Hello all I have a MacPro 2009 that came with a GT120 I ended up getting a GTX285 and had the other one in the computer for awhile and then took it out now a few years later I wanted to put the 120 back in and use a third monitor I read that it can be done but when I put the 120 in the computer won't boot up it just freezes. I'm running everything current Mountain Lion any suggestions would be appreciated.

Thank's