I agree with the people who have said one of each. Single tones are notoriously hard to locate directionally. This is why some ambulances/emergency vehicles incorporate bursts of static (the easiest to place in your field of hearing) into their sirens.
Unfortunately, static would by its quality of randomly changing frequencies contravene the bit of the MOT that says a horn has to be a constant tone (yet another case of legislation/beauracracy getting in the way of common sense
), so using if for your horn is a no-no.
If you really wanted to make it
really noticeable, you could use 4 horns, 2 for high and 2 for low, but detune one of each pair so that they are dissonant. It'd be both easy to work out where it was coming from and so unpleasent sounding that even the most comatose cager could spot you.
