function to get secondary thumbnail

Hi,

Is there a simple way of modifying the function below to call the secondary featured image?

if ( has_post_thumbnail() ) {
			wp_localize_script( 'backstretch-init', 'BackStretchImg', array( 'src' => wp_get_attachment_url( get_post_thumbnail_id() ) ) );
		}

Any assistance would be much appreciated.

Thanks
Darrell

https://www.ads-software.com/plugins/multiple-post-thumbnails/