Once you reach the level where you can use all four turtles at once, you must 
begin building a relationship with your brothers. You can do this by using your 
brother throw, but take turns who is doing the throw. For example, Leo throws 
Ralph and vice versa, Don throws Mike. Successful brother throws will build 
their relationship, and then you can use their double team attacks. However, 
failed throws will make them mad at you, and they will go away and you must win 
their trust again. Raphael seems to be the one that you make mad most often.
To get an "A" in working with your brothers, use the turtle's special move when 
fighting. Keep switching turtles and do the same thing until all enemies are 
defeated.