http://digital.ni.com/public.nsf/allkb/6985219C76DB128C862571DC005CB1CC
"
Problem: Can I acquire from two USB cameras simultaneously with NI-IMAQ For USB Cameras 1.0?
Solution: NI-IMAQ For USB Cameras 1.0 does not
support simultaneous acquisitions from more than one camera (stated at
the top of page 6 in the IMAQ for USB Documentation).
Note that you can still use the software with multiple cameras, but
multiple cameras cannot simultaneously acquire. You can use the IMAQ
USB Enumerate Cameras function to identify each camera plugged into
your system."
Well, i've to go with another solution...