Its a general rule of thumb to only have a second group of Mafia when the game is aimed for advanced players. A second Mafia could screw it up for those at are just learning how to play the game.
If you are interested in moderating a game of mafia, it is highly recommended that you read though this guide.
I imagine that you have come to this thread because first, you're interested in moderating your very own game of Mafia. Well, I'm going to teach you how to be one, and how to be an...