Ubuntu – Dual monitor workspaces in Ubuntu 18.04

Here's a script that switches to the next workspace, wrapping back to the first after the last. Note that workspaces are numbered from 0, maybe this is what threw you when you tried writing a script.

#!/bin/sh
total=$(wmctrl -d | wc -l)
current=$(wmctrl -d | sed -n 's/^\([0-9]\+\) *\*.*/\1/p')
if [ -z "$total" ] || [ -z "$current" ]; then
  echo 1>&2 "$0: Could not obtain workspace information!"
  exit 2
fi
target=$(($current+1))
if [ $target = $total ]; then
  target=0
fi
wmctrl -s $target

--copied the answer from an edit the OP made to their question --

You find the solution here: http://gregcor.com/2011/05/07/fix-dual-monitors-in-gnome-3-aka-my-workspaces-are-broken/

The desired behavior is activated by executing

gsettings set org.gnome.shell.overrides workspaces-only-on-primary false

in a shell.