The reason it only recognizes one is because Mac's OS can only recognize one audio card/device at a time, in this case it's your 900NXS. You have to create an aggregate device, which is essentially a means for Mac OS to utilize multiple audio interfaces/devices. I believe Pioneer has a downloadable program that will handle this for you. Once you've created an aggregate device that consists of your two 900NXS, next time you open Traktor and go to preferences you'll find a nice Audio Device that consists of both your CDJ's.
As far as I know, the aggregate process should be done prior to opening Traktor.