what can a 902 / 135 do better than the as-350b3
It is all about mission profile.
NYPD have been looking for a long time ... well over a year. Last year they were looking at singles including the Koala.
I can only assume that all the time they have been looking has led them to identify a low level urban mission that would be better served with two engines. One I am aware of is night-time 300 feet over the Central Park trees.
Ok there is only one gearbox, rotor etc.. but when low down an engine run-down is far more embarassing than at a height where they can choose a spot to land.