Populate second node reference field based on what first node reference field is selected
You can use custom coding to achieve this, here the idea


Add a script to call a ajax call back on change your first node reference select list.
Define the menu callback in your custom module.
On change of first select list, get the selected node id and pass to above defined function
Write the query to get depended node ids
Output it as 

$options .= "nid." ".$selected." >".$data->node_title."";
     echo $options;
In your jquery ajax success part, add it eg: 
success: function(data) {
f(data){$('#your-second-noderefernced-field').html(data);}


Hope this idea help you to proceed on