Two layer navigation

I’m having an issue trying to create a two layer nav, I want it so that if you click on a page you will be able to see its sub pages, only the main parents should appear on all pages

This is what I have so far, of course this doesn’t seem to work:

<ul>
              	<?php
				if(  $post->post_parent ) {
				  $children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
				} else {
				  $children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
				}

				if ($children){
					$output = wp_list_pages('title_li=&link_before=<span></span>&child_of='.$post );
                }else{
					$output = wp_list_pages('depth=1&link_before=<span></span>&title_li=' );
				}
                ?>
              </ul>

I copied this from somewhere, but wouldn’t mind using any code that works, as you can see from the above the links need a span tag as well

Hope someone can help me