How do I merge this code?
Just merge like:

function mytheme_theme($existing, $type, $theme, $path) {
  $items = array();
    $items['user_register_form'] = array(
    'render element' =&gt; 'form',
    'path' =&gt; drupal_get_path('theme', 'mytheme') . '/templates',
    'template' =&gt; 'user-register-form',
    'preprocess functions' =&gt; array(
       'mytheme_preprocess_user_register_form'
    ),
   );
   $items['comment_form'] = array(
    'render element' =&gt; 'form',
    ),
   );
 return $items;
}