Bridge and Tunnel

"...two couples sitting in front of me - one sixty-something, the other fortyish and very Bridge and Tunnel. "

Is it some sort of geographical hint indicating that they came from somewhere outside of Manhattan or ...?

Yes, you are correct.
New York City is broken up into 5 distinct geographical and political sub-divisions called boroughs--Manhattan, Bronx, Brooklyn, Queens and Staten Island.
Manhattan is an island and can only be entered via a system of bridges and tunnels.
So if someone in Manhattan describes a person as being "very bridge and tunnel" then that Manhattanite is stereotypically talking about someone from the outer boroughs, Long Island or New Jersey.