Assuming there is no issue with 24v coils, which I don't think is what you are asking about, it's quite simple.
In begin.vi, when you open a solenoid, you create a constant for the output Solenoid 1-8.
There is a terminal right above that that lets you create a constant with the number of the card you want. It defaults to module 1, so often it is not wired. Just add the constant for the second card.
This is the first time our team has ever used so many pneumatics components that we needed a second card as well.
