Create a directory for the configurations
mkdir -p /etc/X11/xorg.conf.d
get the identifier from x11-apps/xrandr
xrandr -q
create the configuration for the monitors in /etc/X11/xorg.conf.d/10-monitor.conf (looks like /etc/X11/xorg.conf.d/50-video.conf is not needed)
Section "Monitor"
Identifier "DisplayPort-2"
Option "Primary" "true"
EndSection
Section "Monitor"
Identifier "HDMI-A-0"
Option "LeftOf" "DisplayPort-2"
EndSection
References: