Yan,
There is no way to synchronize the FP IO modules between them as indicated in this tutorial:
http://zone.ni.com/devzone/conceptd.nsf/webmain/2124F6BBD29663D886256CB80054DBD7?OpenDocument#4
But you can control these two instruments almost simultaneously minus propagation time in the serial line and small delays between FP modules.
Regards,
Cyril Bouton
Applications Engineer
National Instruments
Cyril Bouton
Active LabVIEW Developper