Thanks for the info, was looking for that too.You have to raise the dissatisfaction (red bar) of a girl - the easiest way to do this is to force her to wake up after being exhausted from H or order others to break up with her after building a relationship. By increasing the red bar the girl gets the option to call a bad guy for 50 coins, with a low red bar only one guy is available but raising the red bar will let to a option to call another bad guy - the girl can then call them both to her location for 3 P action. Just don't raise the red bar to the maximum, it will cause the girl to leave your game and reset her stats.
Also worth mentioning is that you don't have to do this with every girl, only with one per location, the other girls are ready to bang the two bad guys too without having to built a relationship first or raise dissatisfaction.
But how do you decrease the red bar after that? I think it goes down after a while anyway but it takes long time. Any quick way by doing some good deeds?