" . . . apparently their were two William McNevins running around NYC at the same time, the same age, with two siblings with the same name."
No. In 1870, it was alleged that serious undercounting took place in New York City. So, in addition to the population schedules produced in June, there was a recount in December creating an additional series of population schedules.
If you look carefully at your 1870 census citations, there is one recorded for the 17th ward and the other recorded for the 17th ward (2nd Enum). What you basically have is two sets of data, most likely for the same household.
Only the second enumeration included street addresses and the EDs changed between June and December, but not the wards. Your man is shown to be at 337 East 13th Street in the 2nd enumeration.
He is listed in the first enumeration as being in the 17th ward, ED 12. This ED was bounded by First and Second Avenues on the east and west & 12th and 14th Streets on the south and north. These boundaries exactly match the boundaries for ED 3 in the second enumeration.
All of this would suggest that you have the same household recorded first in June, then again in December.