How to bind workspace to shortcut, and also bind return to previous workspace to the same key
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.