extending Gnome workspace of Fedora 17 to two screens

After having the external screen working in Fedora 17 with the NVidia
driver, the workspace is only on the Laptop display.

I found an option here.

The setting is:

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

After setting this, the workspace is expanded to both screens and can be
switched.