What version of MacOS and catalyst are you using?
The latest Decklink drivers (from their website) certainly work ok using 10.6.8 or 10.8.5 with 2x IntensityPro PCIe cards.
You can use the blackmagic software to test that the system sees both cards and that the inputs are configured in system preferences first
Ensure that you close any other software before starting catalyst (and/or reeboot)
In catalyst, once you enable video inputs, just select the relevant format for each input an it should work
Beware that decklink products are VERY picky about the EXACT format and refresh rate...
A scan converter can be useful when testing !
Alternatively, Active Silicon Phoenix D20PE4H will capture up to 1080p25 or 30 if the input settings are adjusted.
Let me know if you need to borrow a Phoenix card to test before committing to hire or purchase
S