Fighting

We have lots of fighting going on with Jack and Nick.  They fight all the time.  Doesn't matter what they are doing, it always ends up with fighting, hitting, kicking and someone crying.  I mean its out of control.  I think they are both guilty in the fighting.  Jack has a huge bruise on his left side of his head from Nick pounding him.  Nick has had injuries from jack too.  Jack has a huge temper.  He gets mad and then throws stuff.  The weird part is then, when Jack needs to go somewhere like a Hockey game or the Park, he doesn't want to go unless someone goes with him.  He says its not fun to go without Nick.  I need some serious help.  I don't even like cats, but they are like a bunch of girls in their fighting sometimes...