I think that possibly the two horns having a high and a low note enables people to better detect the direction the sound is coming from, high pitch sounds are not very good at indicating the source of the sound. The low notes are better at indicating the source and even when they are sounding at the same time the high and low notes are detectable to our ears even if they appear to be one sound. Low notes can be felt as well as heard.

This is why more and more emergency vehicles have alternate high and low sirens. You know the direction of the sound source much quicker, this is very important if you hear a siren while you are inside your car. Next time you hear these sirens check it out by comparing the directional input you get from the two tones.

Thats what I heard anyway.