I am wondering whether it is possible to combine digial channels from two cards into one task?
Yes, as long as it's all Digital In or Digital Out, the task can have multiple channels coming from two cards.
I think that the following KnowledgeBase article is pertinent to the discussion: Synchronizing DSA, S Series, and X Series Devices With a Single NI-DAQmx Task. So, if your two cards are in the same PXI chassis, and you are performing the same task (DI or DO), you could organize the channels from both cards into one task. Note that you can't do both DO AND DI in one task (but then again, you wouldn't have been able to do that on a single PXI card either). Hope this helps.