In theory - you should be able to use 2x intensity PRO cards with current decklink drivers...
However - due to issues with quicktime video input - this may not be the case:
You should also run your 64bit MacPro in 32bit mode for the moment...
See other posts re quicktime video input / 64bit / graphics cards / 2010 MacPros:
General:
http://chaldee1.gotadsl.co.uk/~richa...ead.php?t=1883
Graphics & 32/64Bit:
http://chaldee1.gotadsl.co.uk/~richa...ead.php?t=1871
Graphics Cards
http://chaldee1.gotadsl.co.uk/~richa...ead.php?t=1875
Quicktime & OS
http://chaldee1.gotadsl.co.uk/~richa...ead.php?t=1781
Simon