A two tone horn is basically two different noises, one quiet and one loud; if you press the horn lightly its very quiet noise e.g. could be used mainly for beeping at your mates (which is for what a horn was not intended - i'm sure it has a more official purpose) or for other minor instances.
Then if you press moderately/hardly you get the default loud noise used for whatever you use a normal horn for.
Hope that explains things; they were and are fitted to higher up the range cars as you've said. In the case of the Punto in the sporting and GT models. They are widely used within F**d especially in the F**d Ghia models.
Matt
can i fit one to my sisters car? 
