you are using two general E series daq cards, for synchronization, there are two options, you can either use external clock and trigger, or you can use the RTSI bus,not all E series is supported by RTSI though.
LabVIEW comes with quite a few god examples about synchronization, if youare suing LabVIEW 6.1, go to HELP menu, click on Find Examples, then click on the Search Tab, type in syn, then you wil be able to see an example "Multiple board synchronization.vi", which should provide you with some good information to get started.
XD Gao
Application Engineering
National Instruments