There is an example that ships with NI-Scope which shows how to synchronize 2 PXI-5112s together. One is configured as a master and the other as a slave. For more info, read the readme.txt which is included with the example. Here is the path to where the example is:
C:\VXIpnp\WinNT\NISCOPE\Examples\c\MStudioC++\5112Sync
Jack Arnold
Application Engineer
National Instruments