second featured post not displaying on page [closed]

You’re trying to get the thumbnail URL to use inside an inline style. You’ll need [the_post_thumbnail_url()][1] to do so. the_post_thumbnail() returns the actual image, not the URL of the image (which you need to display it as a background image.

This particular plugin provides the get_post_thumbnail_url() function to do so.

You would use it like this (including the background-image property):

style="background-image: url('<?php MultiPostThumbnails::get_post_thumbnail_url(get_post_type(), 'secondary-image'); ?>');"

Also, note that if you’re calling the the_post_thumbnail() function outside the loop you will need to provide a $post_id.

<?php
$attr = array(
    'id' => get_the_ID(),
);
MultiPostThumbnails::the_post_thumbnail( get_post_type(), 'secondary-image', $attr );
?>

See the_post_thumbnail() in the WordPress codex, and the multi post thumbnails plugin Wiki and source code to see their implementation.