synchronizing several PCIe-6535B

Hi

I need to synchronize all DOs in four of the PCIe-6535B. They are all connected with the RTSI cable, operating at 5MHz sample rate. 

All 128 lines need to start outputing the data when a single PFI trigger is received. 

What would be the preferable way of synchronizing these four devices? The NI examples show master/slave concept, but is there any better way? I would like to see all of the four devices as one array of 128 channels.

Can I combine all of the lines into a single task and write the data to all of them at once? 

thank you