Dunno if you can use 2 SoundCards on Cross DJ at the same time(is that the software that you have?)
-ASIO 4 ALL drivers(if thats what you are trying) can only be used on 1 SC device at a time.
-One SC is the way to go to avoid problems with latency, compatability, internal clock clashes.....ect Have a deeper look in the forum for more info
Cheers