Hi Mariano,
do all the boards reside in the same pc?
In yes, you can use the same task to acquire from two devices (if the devices support that). If this functionality is not supported, you have to manage two parallel task.
If the boards reside on two different pc, you have to implement some kind communication between the PCs.
You can find some basic examples of DAQmx programming for C here: C:\Users\Public\Documents\National Instruments\NI-DAQ\Examples\DAQmx ANSI C
Hope this helps.