http://www.netrover.com/~timt/cheats/d/doubledragonii.html (Amiga Plus Extra No. 5/97, 05/1997)Start a two player game, then press both fire buttons and the "ESC" key. You will now have unlimited lives.
On the title screen press 'ESC' then type "YEP, I CHANGED THE CHEAT MODE ON THIS VERSION". Now press 'RETURN'. Start the game and you will be playing upside down.