I was unable to find any built-in method for that in cakePHP, you can use simple Jquery function to achieve that functionality.
Here is the code
$someVariable = $("#dropDown-1-d option:selected").val();
In my above example, I am assuming that the values I want to be selected have same 'value' attribute. for e.g.
if value="1" is selected from 1st dropdown then select value="1" for 2nd dropdown as well.
Change the selection criteria as per your requirement.