How to sync brightness of main display with secondary

• Install the binary to /usr/local/bin/ddcctl
• Run

mkdir -p ~/Library/LaunchAgents
curl -o ~/Library/LaunchAgents/com.zerowidth.launched.sync-brightness.plist http://launched.zerowidth.com/plists/3902c360-bb27-0139-1649-5da0311d64fa.xml
launchctl load -w ~/Library/LaunchAgents/com.zerowidth.launched.sync-brightness.plist

This will create an agent which will sync the brightness once per minute.

You can create & configure your own plist using following script:

/usr/local/bin/ddcctl -d 1 -b $(\
  ioreg -c AppleBacklightDisplay \
    | grep \"brightness\" \
    | sed -nE 's/.*"brightness"={"min"=([[:digit:]]+),"max"=([[:digit:]]+),"value"=([[:digit:]]+)}.*/scale=10; \3\/\2*100/p' \
    | bc\
)