[Plugin: Multiple Post Thumbnails] Finding position of attachment in array

I’m using the Multiple-post-thumbnail plugin in order that I can use one of two featured images for display on the post itself, and the second for an image slider on the home page which links to the post.

The stock featured image is always the first attachment in the array, but the second image doesn’t have a fixed position in the array, and I need to extract it but can’t figure out how to do so. Below is the code which currently uses the image at index 0 of the array if there’s no secondary-image, and the image at index 1 if there is. I need this to use the secondary-image if there is one, and the image at index 0 if there’s not.

Anyone offer any assistance on this?

Thanks very much in advance,
:Steve

<?php
		$args = array(
			'post_type' => 'attachment',
			'numberposts' => -1, // bring them all
			'post_status' => null,
			'orderby'=>'menu_order',
			'order'=>'ASC',
			'post_parent' => $post->ID
		);
		$attachments = get_posts($args);

		if ($attachments) {
			echo "<li>";

			if (class_exists('MultiPostThumbnails')&& MultiPostThumbnails::has_post_thumbnail('post', 'secondary-image')) {
				$attachment=$attachments[1]; // second one
			} else {
				$attachment=$attachments[0]; // first one
			}
			echo "<a href='".get_permalink()."'>";
			echo wp_get_attachment_image($attachment->ID,'Full');
			echo "</a>";
			echo "</li>";
		}
	}
	//Reset Query
	wp_reset_query();
?>

http://wordpress.org/extend/plugins/multiple-post-thumbnails/