im not sure if i got it right: you want the cube but only with two sides? correct?
on the gnome-panel is the workspace-switcher, if it aint there add it (right-click on the panel and choose add to panel. scroll down and choose workspace-switcher). right-click on the workspace-switcher and choose preferences.
set the rows to 2. that should give you a two-side-cube. how to go on with the configuring you need to try.
i hope it will work, if i boot in gnome and its different i need to edit the post