have you tried one card per time? Do they work both ones (and both channels) using them as single cards? Is your mainboard one of the suggested ones (or using X58/X79 chipset).
With recent drivers I can get multiple card setups working except with Decklink Extreme 2. With 9.5 (and 9.5.2)I can use in the same PC 2x Decklink STudio 2, 2x Intensity Shuttle and 1 Decklink HD Extreme 2 (I have another one, but the second One is not accessible by Mediaexpress and any other software).