|"My board actually allows me to do it".|
How do you know the above? Looking at the features for the board by Intel doesn't state that. See the link below.
In the cases I mention above, the 2 GPUs acted as 1 and you only used the outputs of 1 GPU to power 1 or 2 displays.
If I understand your question, you want to use outputs from both? I would have to defer to the motherboard manual in that case.
I think you will find that when you install an add in card that your integrated graphics will stop working. The safe way to find this out is to connect 1 monitor to ONLY the integrated graphics while you have the add in card installed. If the integrated continues to work then you should shut down and switch the monitor to the add in card. If it also works then try monitors on both.