No, it isn't. All it would take is the metal drive chassis slipping and shorting something out on another drive. If you are going to stack and operate bare drives (done that), at least slip an antistatic bag between the two drives to act as an insulator.
Yes, the slightly, very slightly, conductive bage will act as an insulator.
Yea, the only problems i've had was when i moved the drives during operation, and the metal of one drive contacted the circuit board of the other.
Vibration is the other thing. under stress the drives may vibrate and exceed their tolerances. secureing them and isolateing them so they don't actually touch. helps.
just don't put them inside the anti-static bag xD these things need to breath too.
2.5" x2 = 5" Not sure how you would be stacking them in one bay?
He's storing the drives, not operating them.