Validation: one of two field required

For CI 4:

PHP Code:

        if($this->request->getPost('field_one') == '' && $this->request->getPost('field_two')){
            $error_msg = 'You need to fill field_one or field_two.';
        }else{
            $validation = Services::validation();
            if($this->request->getPost('field_one')){
                $validation->setRule('field_one', 'label 1', 'YOUR RULES HERE');
            }
            if($this->request->getPost('field_two')){
                $validation->setRule('field_two', 'label 2', 'YOUR RULES HERE');
            }
            if($validation->withRequest($this->request)->run() == false){
                $error_msg = $validation->listErrors();
            }else{
                /* Here your success code */
            }
        } 


Where $error_msg contains your error.