This could be a couple things - is there any text on the blue screen? That might help us figure it out.
It could be a RAM issue - that is the most common error that causes blue screens - how are your timings, and all that in the bios - did you change anything recently? If not, then it is likely something to do with the cards and/or drivers. Do you have old drivers in there? Did you do a complete uninstall of the old drivers first, before installing the new ones? Also, it works better if you get one of the cards in there working good, before you put the other one in (for crossfire or just crunching on two).