Now, thinking logically about this. Does queen pheromone rise or fall?
Warm air tends to rise so I think I might generally unite with Queenless on top, some top ventilation and expect they all smell the same by the time they have read the newspaper and chewed it up.
This year was icing sugar. Works just as well.
Alternatively if I am moving a colony (3 mile rule) and uniting it would be dropped on the other one (less mesing around), so either way.
At least on this one you will only get 2 direct answers but there may be loads of different reasons why. I simply take each event on it's merits.
Regards, RAB