Do the dog parks have a water hose, or spigot. I've always heard the best way to break up a dog fight is with a water hose sprayed at the dogs. Suddenly they lose their aggression and are too concerned about not getting wet. If that's not handy, then maybe just a can of mace or pepper spray might do the trick without harming either dog seriously. Amazon has the dog walker special with spray approved for use on dogs.
Amazon.com : Dog Walker Bundle: Mace Muzzle Pepper Spray - EPA Approved for Animals - Lot of 2 : Mace : Pet Supplies