The KnowledgeBase listed above is correct. The main point to consider is that the DSA cannot except external sources, so it will have to be the "master" and the S series board will have to sync to its scan clock. The NI DSA devices are programmed using NI DAQ function calls and there are many shipping examples in LabVIEW or CVI that synchronize two boards together. Most of these examples use a RTSI to route the clocks, but the PXI backplane can be used in the same manner.
Best Regards,
Jeremy R.
NI - Applications Engineer