Hi Omi: I think, that you need a jQuery Load http://api.jquery.com/load/ something like in the view: jQuery.load( "url to echo function", {params, like category id, if necesary}, '#jquery selector of second combo' )
In the echo function: validate that you get a valid category id do a select with this categery id, and save result in some variable. for each row of the result, iterate to create the <options>, inside a <select> return XML (html_string or SELECT with OPTION) firebug or similar in chrome, will help you analizing the echo response. HTH
