Just need the one cable, Displayport preferably as this cable allows you to use Adaptive-Sync (Freesync) with Nvidia cards which Hdmi doesn't support. Usually manufactures include only one cable. I suppose Asus felt generous if both cables were included in the box. For dual monitors, you'll need another monitor, and then use the Hdmi cable
So disconnect the Hdmi cable and enable Adaptive-Sync in the monitor OSD and Gsync should be available in NvidiaCP if not already. Afaik, Adaptive-Sync on Nvidia cards is still limited to Displayport.