Find active screens with

xrandr | grep " connected"

Example output

HDMI-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 527mm x 296mm
eDP-1-1 connected primary 1920x1080+0+0  (normal left inverted right x axis y axis) 294mm x 165mm

Set desired screen brightness. Range is from 0-1

xrandr --output eDP-1-1 --brightness .5