Are two horns better than one?

My son's '74 Dart Sport and my '76 Duster parts car both have only one horn, mounted underneath the battery tray. Other cars I have seen or pictures posted here on the web have two horns, some mounted on the right side of the radiator.

Is this some type of two-tone (high/low) approach to being heard better than one tone? They look the same, so are the horns adjustable? In other words, can I add the Duster horn to the Dart and make an adjustment to get the two-tone effect?

Just curious,
Jerry