If you get one or the other, you get a beep. If you get both, you get a horrendous warble that hurts people's brains. That's basically how they work, dissonance is more penetrating and attention-grabbing than a single note.
Had some fun with them on the Egg Run at the weekend... Loads of people beeping away with their feeble stock horns, then HHHHHOOOOOONNNNNK from me
