iac,
Sorry, there is no such psak. nor does it make sense, as per Rav Moshes defining the parameters of 600K and a city, regarding Eruvin. If there is nothing separating one from the other it is considered one city and one entity regarding eruvin.
Perhaps we should differentiate by County or State. Where do you draw the line?
I did see regarding Brooklyn and Manhattan that we treat them as separate entities, being that the river separates them.