make a second plank for your second monitor

make_second_plank.sh

#!/bin/bash

plank -n dock2 &
kill $!
printf "Monitor: "
read monitor
echo "Using $monitor as monitor"
gsettings reset-recursively net.launchpad.plank.dock.settings:/net/launchpad/plank/docks/dock2/
for key in $(gsettings list-keys net.launchpad.plank.dock.settings:/net/launchpad/plank/docks/dock1/); do
    val=$(gsettings get net.launchpad.plank.dock.settings:/net/launchpad/plank/docks/dock1/ $key)
    gsettings set net.launchpad.plank.dock.settings:/net/launchpad/plank/docks/dock2/ $key "$val"
done
gsettings set net.launchpad.plank.dock.settings:/net/launchpad/plank/docks/dock2/ monitor $monitor

mkdir -p $HOME/.config/plank/dock2/
rm -rf $HOME/.config/plank/dock2/launchers
ln -s $HOME/.config/plank/dock1/launchers $HOME/.config/plank/dock2/launchers

killall plank

Thanks for sharing, this is super useful!

If anyone reading this wants the script to auto-detect the non-primary monitor, replace:

printf "Monitor: "
read monitor

with:

# get the non-primary monitor
monitor=$(xrandr | grep " connected" | awk '$3 !="primary" { print $1 }')

(if you have more than 2 monitors, you might need to do a loop, instead)

The only thing this script didn't do, was set dock2 as enabled. You can pull the current array (as long as you have at least dock1) and simply append to it, without disturbing other docks, by adding this after gsettings set net.launchpad.plank.dock.settings:/net/launchpad/plank/docks/dock2/ monitor $monitor :

current=$(gsettings get net.launchpad.plank enabled-docks)
append=$(echo $current | sed "s@dock1@dock1','dock2@g")
gsettings set net.launchpad.plank enabled-docks "$append"

Thanks for this, was exactly what I needed. Also thanks @angela-d, your modifications worked spot on.