One way it is possible is to create a new page with the second select list that is filtered by the submitted form element which is the datepicker.
Then use framework builder to add the select list to the page as a plugin inside a div with an ID.
Then use the datepicker onchange event and add the behavior: "webassist->Framework->load plugin to div" to submit the form and reload the plugin which would then be filtered by the first list.