Show Multiple Size URL of one Featured Image/Thumbnail?

no problem to have 2 or more post thumbnails of different sizes.
but what is the “ref” attribute ? do you mean “rel” ?

anyhow – example from the :codex

 <?php 
 if ( has_post_thumbnail()) {
   $large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id(), 'large');
   echo '<a href="' . $large_image_url[0] . '" title="' . the_title_attribute('echo=0') . '" >';
   the_post_thumbnail('thumbnail');
   echo '</a>';
 }
 ?>

mind that the_post_thumbnail() will give you the full HTML for the image , to get other attributes, you will need to use get_the_post_thumbnail() or get_post_thumbnail_id() with wp_get_attachment_image_src

    $fullsize = wp_get_attachment_image_src( get_post_thumbnail_id(), 'fullsize' );
    $fixed_height = wp_get_attachment_image_src( get_post_thumbnail_id(), 'fixed_height' ) ;
// returns array in which :
echo $fullsize[0] ;// url of fullsize 
echo $fixed_height[0] ;// url of fixed_height