As simple as this:
<select name="first" onchange="this.form.second.selectedIndex=this.selectedIndex">
This assumes that the options in the two <SELECT>s are related element by element. If it's more complex than that (e.g., maybe the values in the second select are arranged alphabetically?), then the JS
code will be longer. But it still won't be complicated.
So if the above doesn't work, show a sample of your *REAL* <SELECT>s.