Change value of second dropdown based on value from first dropdown

I have two dropdowns on a form
FirstDropdown, values (One,Two,Three)
SecondDropdown, values (1,2,3,4,5)
I need to add 6,7,8 to the second Dropdown which i can add to that Dropdown values. But the requirement is that those newly added values should only be visible if One and Two is selected from the FirstDropdown. What will be the best approach to accomplish this requirement