Found here:
https://blog.summercat.com/configuring-mixed-dpi-monitors-with-xrandr.html
Basic working command:
xrandr --dpi 276 --fb 7040x3960 \
--output eDP1 --mode 3200x1800 \
--output HDMI2 --scale 2x2 --pos 3200x0 --panning 3840x2160+3200+0