Hello Sebastian
first of all I'll need some more information about your intention. What exactly are you going to do? Do you want to connect, or divide signals? (Which signals?)
Maybe if you have a look in the manuals of the cards, you'll get closer to this. Here you can find out the pin assignment of the cards.
Manuals and specification:
PXI-6723:
http://www.ni.com/pdf/manuals/370822c.pdf
http://www.ni.com/pdf/products/us/04-3513-301-101-DLR.pdf
PXI-6259
http://www.ni.com/pdf/products/us/044063301101dlr.pdf
http://www.ni.com/pdf/manuals/371291h.pdf
Regarding
Gregor Allexi