[Plugin: Multiple Post Thumbnails] get_post_thumbnail_id()

You can use MultiPostThumbnails::get_post_thumbnail_id() to get it. The arguments are: post type, id used to register the thumbnail, and the ID of the post you want to get the thumbnail ID for.

e.g. MultiPostThumbnails::get_post_thumbnail_id('post', 'secondary-thumbnail', get_the_ID());