This page demonstrates how to use Javascript to:
1- set a select box value based on a previous select-box value
For this example, if you select "Two" in the frist drop-down, the second one will also be set to "Two". Any other choice for the first select box results int he value of the second dropd-won being set to (blank).
To see how this works, view the source of the page and look at the code between "Begin Sample" and "End Sample"