As said HeadKaze,
If you are using mame you use the Ini way or open you .cfg with AtomicManager ,
then go on the "Folder" tab
then go on "main" subtab
and just behind your mame.exe in the Default Emulator Field add , for instance : -rol to have something like :
c:\mame\mame.exe -rol (or -ror)
FYI , i have an official forum for support in English here :
http://spaceinvaders.7.forumer.com/ in AtomicFE section.
As you have a coktail cab, you may be interrested in the Coktail Mode option , to activate it, go on "options" tab , "option 2" subtab , and tick "Enable cocktail mode" . In this mode when you press "PLAYER2 BUTTON 2" , the screen will be flipped to the second player and control are given to him to allow him to select a game.