Describe the suggestion
Simple as it can go. Would be a nice small implement to have a secondary horn or whistle for engines that have either a two tone horn or whistle. Just adding the new sound and key bind, as well as having the Whistle Control Widget able to set up for Whistle 1/Horn 1 or Whistle 2/Horn 2

Applications of the suggestion
A fair amount of British Diesel Traction has Two Tone horns, meaning two independent tones that can be toggled separately. As well, the Great Western Railway had two tone whistles that acted independent of each other. This also opens the opportunity for those who want to make a secondary horn or whistle for the engine for the sake of diversity.