[Theme: Twenty Eleven] Undefined Function In Theme

I could be wrong, but I believe that is_multi_author() is a function introduced in WordPress 3.2.

It’s trivial to add this to /wp-content/themes/twentyeleven/functions.php

function is_multi_author() {
  global $wpdb;
  $rows = (array) $wpdb->get_col("SELECT DISTINCT post_author FROM $wpdb->posts WHERE post_type = 'post' AND post_status = 'publish' LIMIT 2");
  $is_multi_author = 1 < count( $rows ) ? 1 : 0;
  return (bool) $is_multi_author;
}

@garymotteram
why don’t you upgrade to wp3.2?

Where does this code go? Should it replace the existing code?

generally, it would be added into functions.php of your theme;
however, i would not advise this, as you are going to get another error message when you later update.