Vilas,
Please refer to
this KB on synchronizing DSA and S series cards.
I would recommend that you one of the DSA boards as the master and export the sample clock to the S series card.
You can check to see if everything is synchronized by sending a signal to a channel on all boards and checking to see if the samples are aligned.
Abhinav T.
Applications Engineering
National Instruments India
LabVIEW Introduction Course - Six Hours
Getting Started with NI-DAQmx
Measurement Fundamentals