In my travels I've come across a couple of facilities that serve as the home to more than one post office. I'll explain. Ordinarily, a post office is an independent building in its own community. Sometimes a post office is outside its community: Trenton, NJ's MPO is technically in "Hamilton Township", though it serves Trenton.
A bit north of Princeton, NJ, there is one building that hosts the post offices for two communities. They used to lie in two separate buildings, about a mile apart, but they've since learned to share. Kendall Park and Franklin Park are two independent post offices possessing different Postmasters, distinct PO box sections, and two different postmarks, though it has only one retail counter. The Franklin Park PM works in a different building and you have to ask the clerk for its postmark from the back. Here's a picture of the building:
I came across a similar, but by no means identical, situation in Westchester County, New York: the Hartsdale post office shares a facility with the Carriers of the Scarsdale post office. The Hartsdale Postmaster has no authority in the Scarsdale half of the building (which is under the jurisdiction of the Scarsdale Postmaster, who works at their main office). Again, each has its own postmark dater.