add_action('acf/save_post', 'my_acf_save_post');

function my_acf_save_post($post_id)
{

	if ($_POST['post_type'] != 'cpt-author')
		return;

	$Post_Title = '';

	$Fields = get_fields($post_id);

	$FirstName = $Fields['ACF-Author-FirstName'];

	if (isset($FirstName)) {
		$Post_Title = $FirstName . ' ';
	}

	$LastName = $Fields['ACF-Author-LastName'];

	if (isset($LastName)) {
		$Post_Title = $Post_Title . $LastName . ' ';
	}

	$post_update = array(
		'ID'         => $post_id,
		'post_title' => $Post_Title
	);

	wp_update_post($post_update);
}