hammer on hammer SHOULD BE fine. Ive come to the conclusion that it depends on the coral..lol. Ive got hammer, frogspawn, and torch touching each other with no probs. (they are all in the same family, and notorious for sweepers). The only one Ive had send out sweepers is my galaxia. Ive seen the same thing asked on RC and the consensus seems to be that it really depends on the individual coral.

You can aim the flow (not too much obviously or you'll blow the flesh off the skeleton) in that general direction and blow the sweepers away from your other corals.

-Christina