In LabVIEW, go to Help>>Find Examples. Navigate to Hardware Input and Output>>Traditional DAQ>>Multiple Device. There is an example there called "Two E Series Shared Scan Clock". That example is for analog input, but you should be able to modify it to be for analog output. Basically you will just have to share the scan clock from one board(master) and use it as an 'external' clock for the other board(slave).
-Alan A.