I agree with @kittythehareIs it possible there's a witness that was present for both incident's? A regular train rider perhaps?
The witness would have needed to be on the same train and in the same car.
Such an occurance would be somewhat plausible if the both riders boarded a very late, or very early train early in its starting point in what I have heard New Yorkers call "furthest Brooklyn" or "furthest Bronx" etc. Maybe regular commuters with the same start time and start point.
But.... as the day progresses and given the sheer size of NYC and the subway system, I think the odds get extremely small.