Xinerama is for multiple displays, i.e. more than one monitor, not for workspaces.
Workspaces are typically handled by the window manager. Some window managers support this, some don't. For instance, I used to use AfterStep and Sawfish, both of which supported workspaces with multiple pages. In other words, you could have four virtual desktops, each subdivided into four viewports and windows one workspace could overlap several viewports. Basically, each workspace could be spread over more than one screen.
As far as I know, the default GNOME and KDE window managers only allow one screen to a workspace. You might want to look into either using a different window manager under GNOME/KDE or switching to another desktop setup.
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