So far the best ideas I've gotten are:
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Grab both dogs by their rear legs
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Splash water on them with a hose/bucket/bottle
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Make a very LOUD sound
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Pepper spray or some irritating spray
Although my dad has successfully broken up fights between our family cat and dog at home by throwing a soda can stuffed with metal nuts, bolts, screws on the floor near the fight.
However this is not an option in the heat of the moment on the street.