$post_tags = get_the_terms( $post_id, 'post_tag' );
$authors =
$remove = array();

foreach ( $post_tags as $post_tag ) {
    if ( preg_match( '/^[[0-9]+] *([^,]+), *(.+)$/', $post_tag->name, $match ) ) {
        $fullname = "$match[2] $match[1]";

        if ( $term = get_term_by( 'name', $fullname, 'author_tax' ) ) // Already exists
            $authors[] = ( int ) $term->term_id;
        elseif ( ! is_wp_error( $term = wp_insert_term( $fullname, 'author_tax' ) ) ) // Create new author tax term
            $authors[] = ( int ) $term['term_id'];

        $remove[] = ( int ) $post_tag->term_id;
    }
}

$authors && wp_set_object_terms( $post_id, $authors, 'author_tax', true /* Append terms */ );
$remove && wp_remove_object_terms( $post_id, $remove, 'post_tag' );