Use plank on multi-monitor setup without creating multiple docks. Simply move the mouse to the bottom of any monitor and plank moves there.
Start from the CLI.
autoplank
Requires Go 1.8 or newer.
go build -o autoplank && sudo mv autoplank /usr/local/bin
You may want to create a service to start and keep running in background for convenience.
$HOME/.config/systemd/user/autoplank.service
[Unit]
Description=Autoplank Service
[Service]
ExecStart=/usr/local/bin/autoplank
Restart=always
[Install]
WantedBy=graphical.target
systemctl enable autoplank --user
systemctl start autoplank --user