How do I use the same function more than once on the same page?

Are the other 15 instances using the same information in the 2nd field? There are a number of ways you can approach this. The more you know about JavaScript, the easier it will be for you. The way I would handle this is to create a document level script with a function that will change the information in the 2nd field based on the first field. You can e.g. pass in the two field names as parameters to the function:

function change2ndField(sourceName, targetName) {
    // ....
}

You can then just call that function with the pair of field names:

// ...
change2ndFieldName("TheFirstField", "TheSecondField");
// ...

For this to work, you need to be familiar with creating JavaScript functions.

If that is not possible, the most straight forward approach (but the one that requires the most typing) is to just duplicate the code for each pair of fields. You just have to make sure that you change any reference to a field name so that the script links up the correct two fields.

Do all of the drop-downs need to populate the same fields, or is it a different one each time?