What type of synchronization do you want between the 6115s and the 5122s? Do you just want to make sure that they all start at the same time? Or do you want to do something more like making sure that sample clock edges are aligned (meaning you can only sample at integer values of each other)?
Also, what triggers the start of the acquisition (hardware trigger, software, etc)? If hardware, which board retrieves the trigger?
Are you working with a PXI or PCI system?