TWO VIDEO CAPTURE CARDS

I HAVE 2 BLACK MAGIC INTENSITY PRO CAPTURE CARDS PHYSICALLY INSTALLED ON A 2009 MAC PRO THATS RUNNING OSX 10.11.15. I INITIALLY INSTALLED THE DRIVERS FOR THE FIRST CARD AND IT SEEMED TO WORK FINE, HOWEVER WHEN I PUT IN THE SECOND CARD I JUST GET A BLACK SCREEN FROM BOTH. IS THERE A SPECIAL WAY THIS MUST BE CONFIGURED? DO I HAVE TO INSTALL THE BLACK MAGIC DRIVERS AGAIN? I DID SOME EXPERIMENTING AND BOTH CARDS WORK WHEN THEY ARE INSTALLED IN THE PCI SLOT THAT THE INITIAL CARD WAS INSTALLED IN, B.U.T IF I TRY ANY OF THEM IN EITHER OF THE OTHER SLOTS I JUST GET THAT BLACK SCREEN AGAIN. CAN ANYONE HELP?