Let the boxing match begin!

I think we need to buy those blow up boxing gloves. Who are the contenders? In this corner weighing in at 30 pounds you have 3 1/2 year old Hunter and in this corner weighing in at a whopping 35 pounds you have 4 year old Aiden! They are the best of friends and the worst of enemies all at the same time. As soon as one of them wakes up in the morning or they are away from each other for any length of time, they are asking for each other. One of the last things Hunter tells me before he goes to bed is that when he wakes up he's going to play with Aiden. But most of the time, 5 minutes into their playing, the boxing starts. I'm convinced you can hear the screaming and yelling all the way back in Ohio! It usually starts over a toy and immediately leads to slapping, biting, punching, kicking, you name it, they do it. And unless you see it for yourself, you never really know who's at fault. Lately it gets so aggravating that we've started telling them if one of them is in trouble, they both get in trouble. We just started this like two days ago so I'm not really sure if it's working or not yet. On the highly likely chance that it doesn't work, anybody have any other ideas? The usual time out and talking to them about what they've done wrong just doesn't seem to be getting us anywhere.