This question is asked a lot lately...
Server side solution: Basically you need to do a form submission back to the server with an onchange event on the select element.
You may want to look at this post:
https://coderanch.com/t/115659/HTML-JavaScript/Select-Box-one-page-second Client Side Solution: Search on google for JavaScript doublecombo. Basically you write out arrays with your datatbase information when the page loads.
Eric
[ February 22, 2005: Message edited by: Eric Pascarello ]