Beat frequency between two frequencies is f1-f2. So two organ pitches 2 hz apart will beat at 2 hz. The celeste on my Hammond is about 3 hz, but don't know anything about pipe organs, alas.
That oracle of all wisdom, the liner notes on commercial albums, says orchestras go to higher pitch for "greater brilliance". I suppose they mean the string instrument overtones, as my bassoon got fewer overtones the more I pinched the reed. I'm sure there are 10000 other opinions. When I fantasized about buying a Heckel bassoon at the factory in Mannheim?(atlas misplaced) they wanted me to pencil in the pitch on the order form. They were sure I could get all my questions answered by "my teacher". I hadn't had a bassoon teacher in 14 years, lacking as I did a bassoon.