I don't think that you can do this. The thing you're asking is inconsistent with the way workspaces and displays are designed in Ubuntu.
I will try to explain why. The curently workspace is, as its name itself says, the space where curently you are. Now, when you connect another display at the machine, the current workspace (and all others) will be expanded in the new display. When you change the workspace in first display, its expansion will change in the second display as well and vice-versa. So, the display is a something like a 'child' of the workspace. In next image you can see how it look:
What you intend to do is something like:
As you can see this doesn't look ok.
Of course, as long as you have unselected Mirror displays in System Settyngs > Hardware > Displays (as in next image), you can view some programs on one display and other programs on another display. Also you can drag and drop windows from one display to another.
You can also to watch this video to understand better: http://www.youtube.com/watch?v=Vl9P9a2L7Q4.