They come in pairs!
A couple of minutes ago, 2 BA 777s flew overhear JFK-LHR, just 2 minutes behind each other. Ahead of them was a United flight arriving from ORD and behind the BA flights was another United flight ex ORD.
I have to admit, it is nice to hear these flights approaching. It looks as if there will be an American from MIA soon and then... quiet skies for probably the rest of the day!