Question about Acer Predator XB273KGP

In order to operate the monitor with 4k & 144 Hz, I have to use 2 Displayport cables. My problem is that the computer thinks it is 2 monitors even though I only have this one connected. If I remove the one cable I can use it normally with 120 Hz. What is the problem? My GTX 1080Ti supports Displayport 1.4