try the following css:
.box > .rel > a.thumbnail:hover ~ h1 { text-decoration; underline; }
and add a class attribute:
<div class="box">
<div class="rel">
<h1><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h1>
<a class="thumbnail" href="<?php the_permalink(); ?>"><?php the_post_thumbnail('homepage-thumb', array('alt' => '', 'title' => '')) ?></a>
<?php if ($mulderamanda_theme_options['images_only'] == 0): ?>
</div>
</div>
ps:
deleted the unmatched </div>
.