Windows 8.1 Desktop / Metro Switching Logic.
Since the 8.1 update I've been trying to figure out how Windows decides what metro "desktops" do when you switch between them using the windows-tab shortcut. At first it seems to be completely random on my non-touch laptop. But after writing this question out I seem to have puzzled out what's going on, so hopefully it will help you out too. It's NOT transparent or intuitive at all with just two metro apps, but here goes. (note that a metro app CAN be the windows desktop)
We start with a discrete two-unit sidelaid desktop (two metro apps stuck together forming a discrete unit) and a third full-screen Pic1). Here, a INSTANT win-tab will switch between the two-unit and the third full-screen app (here, the desktop). (note that Pic2 shows the discrete two unit as one "desktop" in the upperleft).
If you HOLD down win-tab long enough for the side-bar to show up in the left (again, see pic 2) it will kick-in a new function (PIC3). The selected app (whether the original desktop, or if you hit tab twice to select a new full app) from the sidebar will now overlay and ask you where you want to put it, left or right.
You can do this with (touch, if available), mouse, or the keyboard :(arrow keys then enter)
If you choose either, now a quick win-tab will quick switch between THAT side app, and the one that replaced it. It will not go back between the NEW two-unit and the desktop.
A third option DOES exist. You can pull the center overlayed app UP (PIC4). and now a quick win-tab will switch between the two-unit and the NEW full size metro app.
So the desktop functions like in windows 8, as a metro app to the win-tab multi-switcher, but there are just new forms that this takes.
I have not experimented with how a 3-unit, or 4-unit desktop (4 metro apps overlayed together) would function under 8.1, nor how a two or three monitor setup would work.