I seriously doubt that there's an agreement to do that. However, I would assume that the schedulers try to make it so that they are split home/road as much as possible.
They don't do it in other cities. Exhibit A would be last season when I went to New York for the Braves vs. Yankees and Orioles vs. Mets day-night, two-ballpark doubleheader.
My dad went to a triple-header last year -- Nats 1 PM game, then the Toronto doubleheader(I went to this). I got the latter for "throwback prices" in the LF seats.