I have 6 labs, all are house dogs, 4 males, 2 females everyone is intact. My one male will start a fight when the girls are in heat.....I watch him like a hawk and sometimes he spends as much time in his kennel as the girls do.
A couple times when I wasn't watching he has started fighting with the other younger male, because it is so alarming, I always jump in and grab them by their collars and usually throw one or both in the corner while yelling NO FIGHTING! I am surprised it has worked and I haven't gotten bit. Someone on here suggested a wiffle bat...I think that is a great idea, I will have one before my girls next heat.
Now that I have learned alittle more though, I look for the signs before something starts, posturing with tail up, ears back, and I will make them break eye contact with each other if I think I see things escalating. But it can happen in an instant if you are not watching.
I think it is the pure adrenaline rush that gives you the strength to break them up.