Co-Authors Plus is a nice plugin for assigning multiple authors to posts. However, I find its template tag for displaying byline rather limiting because sometimes we want to use fancy markup for the byline (think of avatars, publish/modification dates, etc). You can use the code below as the base for your theme’s byline markup (in this example, we’re overriding twentyfourteen_posted_on()):

function twentyfourteen_posted_on() {
	if ( is_sticky() && is_home() && ! is_paged() ) {
		echo '<span class="featured-post">' . __( 'Sticky', 'twentyfourteen' ) . '</span>';
	}

	// Set up and print post meta information.
	printf( '<span class="entry-date"><a href="%1$s" rel="bookmark"><time class="entry-date" datetime="%2$s">%3$s</time></a></span>',
		esc_url( get_permalink() ),
		esc_attr( get_the_date( 'c' ) ),
		esc_html( get_the_date() )
	);

	// get the co-authors
	if ( function_exists( 'get_coauthors' ) ) {
		$authors = get_coauthors();
	}

	// Fallback to WP users
	if ( empty( $authors ) || ! is_array( $authors ) ) {
		$authors = array( get_userdata( get_the_author_meta( 'ID' ) ) );
	}

	foreach ( $authors as $author ) {
		$_args = apply_filters(
			'coauthors_posts_link',
			array( 'href' => get_author_posts_url( intval( $author->ID ), $author->user_nicename ) )
		);

		printf(
			'<span class="byline"><span class="author vcard"><a class="url fn n" href="%1$s" rel="author">%2$s</a></span></span>',
			esc_url( $_args['href'] ),
			esc_html( $author->display_name )
		);
	}
}

“So how do I display the avatars” you ask? Add the following code to your theme’s functions.php file and use it to get the avatar image instead of the default get_avatar():

/**
 * Wrapper for get_avatar
 *
 * @param mixed  $user    User object, ID or email
 * @param int    $size    Avatar size
 * @param string $default Default avatar image
 * @param string $alt     Alt text
 */
function kucrut_get_avatar( $user, $size = 33, $default = '', $alt = false ) {
	if ( is_object( $user ) && function_exists( 'coauthors_get_avatar' ) ) {
		return coauthors_get_avatar( $user, $size, $default, $alt );
	}
	else {
		return get_avatar( $user, $size, $default, $alt );
	}
}