如果添加第二个角色的内容是通过标准方法进行的：WP_User::add_role() 然后是add_user_role 挂钩：

/**
 * Do something whenever a new role is added to an existing user
 * 
 * @param String $role
 * 
 * @return void
 */
function wpse327081_add_uer_role( $role ) {

    // Do Things

}
add_action( \'add_user_role\', \'wpse327081_add_uer_role\', 10 );