first, i know this situation sucks. i have been in it myself.
there is no perfect answer.
the best ideas i have come up with include things like
a) putting up information on violent relationships, available services, self-defense classes, etc on flyers around the area, or in pamphlets in local cafes, laundromats, etc. obviously this is so that the people involved can find it and use it to help themselves.
b) trying to speak to the person who you think is the most receptive when (if) you see them on the street (speaking to them as non-judgmentally as possible, about friends they might have who might help them -- assuming they want help -- or what they think about their relationship). this is extremely unlikely to be helpful, unless the timing is really lucky, or unless you have the opportunity to actually get to know them. hardly ever happens, i expect.
obviously none of these things are options to interrupt an actual violent event. interrupting something is a lot more difficult than trying to keep it from happening again. and without knowing the people and their situation, interruptions are extremely unlikely to be helpful in the long run.
good luck. i know it's a hard position to be in.