So these commands are what I use:

nvidia-settings --assign "CurrentMetaMode=DPY-4: nvidia-auto-select @1920x1080 +1920+0 {ViewPortIn=1920x1080, ViewPortOut=1920x1080+0+0}, DPY-1: nvidia-auto-select @1920x1080 +0+0 {ViewPortIn=1920x1080, ViewPortOut=1920x1080+0+0}"

nvidia-settings --assign "CurrentMetaMode=DPY-4: nvidia-auto-select @1920x1080 +0+0 {ViewPortIn=1920x1080, ViewPortOut=1920x1080+0+0}"

I use fluxbox and xrandr.