The simplest thing is to inspect the autocomplete widgets available at
script.aculo.us and adapt them for what you need.
I guess you’d have the second dropdown disabled until a company was
selected. So attach an onComplete handler to your first dropdown which
enables and configures the second dropdown to get employees for only the
selected company (or something like that, there are many variations).
I’ve adapted my own version of the script.aculo.us Ajax.Autocompleter
recently, and it is a very good starting point for constructing your
own.
http://demo.script.aculo.us/ajax/autocompleter
Cheers,
Jonathan