Though this may be theoretically possible, one practical consideration is that it is unlikely that both drives will spin at the exact same speed simultaneously (i.e. as the sound from the one drive would not be of equal frequency and 180 degrees out of phase w/sound from the other, the noise from the two drives would not cancel). For more info, see this
thread. Take care.
PS Please note that I am far from being an expert in physics.
PPS Also, for this to have a chance at working, wouldn't the two drives have to be oriented not only upside down but one would have to be backwards as well (i.e. the area for connecting cables would the face front of the case; incidentally, a friend of mine oriented all of his hard drives backwards in the name of cable management and it worked quite nicely)?