Basically I imagine it like this: I am on workspace 5, I press "shortcut" and it moves me to set workspace number 8 (or something). When I am done, I press the same shortcut and it moves me to workspace 5 again.
System Preferences > Keyboard > Shortcuts sets keyboard shortcuts for all Desktops to
CTRL n (where n is the number of the Desktop).
However, they are disabled by default. So it would be very easy to press
CTRL 8 and
CTRL 5 to switch between Desktops. If you wanted the same shortcut for both actions, you would have to use some kind of script to check the current Desktop setting and make the appropriate move.