Two select boxes - can first select box control the ipOpts of the second?

I am using two select boxes. Now I want that the ipOpts-Fields of my second select box change depending on selecting a value in the first select box.
The table is populated with an Ajax-Request on loading from TableLoadServlet. The first select-box is filled also on loading the table with the ipOptsArray. So far my program is working. Now my problem is how can I find out, which value is selected in the first select box and then how to load the ipOpts data to populate the second select box. What command should I use to send to the server?

Here is my code:

$(document).ready(function() {

var editor = new $.fn.dataTable.Editor( {
"ajaxUrl": "TableLoadServlet",
"domTable": "#timesheet",
"fields": [
{
"label": "FirstSelectBox",
"name": "project_number",
"type": "select",
"ipOpts": ipOptsArray
},
{
"label": "SecondSelectBox",
"name": "pos_name",
"type": "select",
"ipOpts": functionToLoadData(project_number.selected.value) // should be refreshed when selecting a value in the first select box
}
]
}

} );