Game must suport cocktail mode, if it dosnt then the only option you have its to play the game on a split screen mode, if the game does support cocktail mode, all you have to do its to co to the settings on mame, then choos the type of cabinet, generaly its on upright so you have to change'it to cocktail, then the game will rotate automatic. To play games on split screen mode you need to go to the video settings in mame ,and choose cocktail, that would split the screen in 2 images .