You'd better know what you're doing if you pick a fight with two large dogs. With aggressive, escalating behavior like these last two guys are recommending, don't be surprised if the dogs bite back. I wouldn't necessarily back down, but force isn't always the best negotiating tool.
Woah I never said aggressive escalating behavior ... just that you should stand your ground and be confident, NOT aggressive. IF the dog comes at you, show it who's boss. But not with hitting or any kind of pain inflicting force. If you don't feel comfortable putting a dog on its back then you are probably feeling pretty nervous around the dog and its going to pick up on that and act like it owns you.
I've been bitten only once by a dog out of the many times I've had a dog challenge me ... and that was the time I ran away. I've been around dogs a long time and connect well with them. I understand my dog better than I understand people most of the time (part of why my name is what it is). Dogs respect confidence in your place in the world ... and thats all I really think anyone needs to know to feel safe around them.