Ok forget that code.
Do not edit the Twenty Eleven theme. It is the default WordPress 3.2 theme and having access to an unedited version of the theme is vital when dealing with a range of site issues. Create a child theme for your changes.
in your child theme content-single.php change this:
printf(
$utility_text,
$categories_list,
$tag_list,
esc_url( get_permalink() ),
the_title_attribute( 'echo=0' ),
get_the_author(),
esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) )
);
to this:
$author = get_post_meta($post->ID, 'customauthor', true);
$authorlink = get_post_meta($post->ID, 'customauthorlink', true);
$author = ($author != '') ? $author : get_the_author();
$authorlink = ($authorlink != '') ? $authorlink : esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) );
printf(
$utility_text,
$categories_list,
$tag_list,
esc_url( get_permalink() ),
the_title_attribute( 'echo=0' ),
$author,
$authorlink
);