OK I found a way of doing this here:
<?
$second_thumbnail_id = get_post_meta(get_the_ID(), "work_second-image_thumbnail_id", false);
$second_id = $second_thumbnail_id[0];
// The thumbnail caption:
echo get_post($second_id)->post_excerpt;
?>
I just had to change Change “page_second-image_thumbnail_id” to “work_second-image_thumbnail_id” which is the name of my post_type.
I hope this helps people out.
On another note it would be great if there was a way of making WordPress convert line breaks entered into the caption into
tags, instead of manually adding
tags at the end of each line in the caption. I guess that’s another topic though.