Exactly. You need two cables connected through the XLR connectors. I don't know if you can find cables with two 25-pin D-sub connectors that will work from one PDI-8 card to the other. I believe that some cross wiring is needed.
What cards are in the HDR? I would expect it to have OPT-8 cards installed.
If that is the case, you should keep one of the PDI-8 cards and put it into the ALT slot in the d8b to interface with external FX gear digitally and sell or swap the other three PDI-8 cards for OPT-8 cards. Fiber optic cable are not that expensive.
Peter Holmquist.