Yes, it's possible,
I did the same operation you need yet, between 2 differents Ni-Daq.
I connect by HARD CONNECTION the OUT counter from PXI-6025 to GATE of 4 conuter on PXI-6602.
I need count with PXI-6602 on 4 ways during high level of conuter 1 and 2 on PXI-6025.
Contact me if you need some detail configuration fro the counter
applications example:
http://digital.ni.com/worldwide/france.nsf/webcustsol/2CA4E93DC3D774E58025694900523020?OpenDocument&node=163040_f