I use a single BENQ XL2720 DVI-D monitor with DVI input. The game video configuration window allows me to select the 120hz option however whenever I open the config window to check settings it has defaulted back to 59Hz. All other settings remain as chosen. My video card is a AMD R9 390
This seems to be some kind of bug, I have reported it. If you open the INI file the settings should be ok (i'm have an nvidia card).