In code you posted you have issue with syntax – you should stick either with braces style or colons style, mixing them is not supported.
if(class_exists('MultiPostThumbnails')
&& MultiPostThumbnails::has_post_thumbnail('post', 'secondary-image')) {
MultiPostThumbnails::the_post_thumbnail('post', 'secondary-image', NULL, 'post-secondary-image-thumbnail');
}
else {
the_post_thumbnail();
}