I’m using a dual monitor setup at work (one for code, one for manuals – or whatever they call it 😉 ) and had the strange issue with the workspaces only applied to the default left screen (the one with the panel).
I tend to hate that default behaviour because it’s really irritating and annoying when you organize your workspaces over multiple monitors and expect the right monitor to switch to the current workspace application layer as well (as known from KDE 4).
Luckily there’s an option to be set as described here (run as the current user in your terminal):
$ gsettings set org.gnome.shell.overrides workspaces-only-on-primary false