See https://live.gnome.org/GnomeShell/DesignerPlayground/MultipleMonitors.

"
This allows different kinds of work:

1) You flip workspaces often in one monitor to do most of your work, while the other monitor remains static. In the static monitor, you keep a pervasive to-do list, web browser, IM program — the things that "you always need to glance at". Currently (in Metacity) I fake this behavior by using multiple workspaces, and a few sticky windows that stay in the second monitor. It works very well for my to-do list and journal.

2) Flip workspaces independently in each separate monitor, so you get as close to having "multiple computers" as you want. 
"

Only 1 is implemented. The request is to implement the 2nd kind and expose it in the UI.