Profile: John

// access any url parameter
// [wpv-post-param var="someparam"]
add_shortcode( 'wpv-post-param', 'wpv_post_param_shortcode' ); 
function wpv_post_param_shortcode( $atts ) {
  if ( !empty( $atts['var'] ) ) {
    $var = (array)$_GET[$atts['var']];
    return esc_html( implode( ', ', $var ) );
  }
}

// combine parent post type relationship filters
add_filter( 'wpv_filter_query', 'filter_both_parent_types', 10, 3 );
function filter_both_parent_types ( $query, $view_settings, $view_id ) {
  if( $view_id == 1234 ) {
    $selected_parent1 = isset($_GET['bookid']) ? $_GET['bookid'] : null;
    $selected_parent2 = isset($_GET['authorid']) ? $_GET['authorid'] : null;
    $args = array(
      'relation' => 'AND',
      array(
        'key' => '_wpcf_belongs_book_id',
        'value' => $selected_parent1,
        'compare' => '=',
        'type' => 'numeric'
      ),
      array(
        'key' => '_wpcf_belongs_author_id',
        'value' => $selected_parent2,
        'compare' => '=',
        'type' => 'numeric'
      )
    );
    // add these arguments to your meta query
    $query['meta_query'] = isset($query['meta_query']) ? $query['meta_query'] : [];
    $query['meta_query'][] = $args;
  }
 
 return $query;
}