Recently, after I updated my whole Arch config, something strange happened. My Nvidia Settings isn't showing an ultrawide option for my second monitor, despite both monitors being identical models. I've even tried manualy changing the /etc/X11/xconf.d file like so:
Option "metamodes" "HDMI-0: 2560x1080 +2560+0 ..." But then I got a black screen. How can I change this?

/etc/X11/xconf.dsince it is now all automatically set, with the exception of possibly the keyboard & mouse. You need to look at the log files to see what it sees.