I am not with Laminar Research; just a flight simmer.
Follow the advice given in the reference link regarding what cards not to use if you want good performance.
I understand that 2 cards can be used with X-Plane but you need to select how they are used. Many current cards have multiple connections, say 3 screens (1 card) for outside cockpit views and the other card for other screens for instruments etc. You may find performance (frame rate) will be down. You need to ensure you have a "good" power supply somewhere in the order of 1000w. Without looking at the card supply power requirements some cards require between 350 and 450 watts for each card.