¿Cómo mostrar los registros secundarios en la fila secundaria de la tabla de datos con la identificación respectiva?

Siguiendo la primera idea, usando una ventana emergente de detalles, el controlador se vería así:

public function Listdetails_ajax(){  
   $draw = intval($this->input->get("draw"));
    $start = intval($this->input->get("start"));
    $length = intval($this->input->get("length"));
    $books = $this->Search_model->getTotalList_of_primary();

    $data['draw'] = 1;
    $data['recordsTotal'] = count($books);
    $data['recordsFiltered'] = count($books);
    foreach ($books as $key => $row) 
    {
        //print_r($row->customer_id);
        $arr_result = array(
                    "member_id" => base64_encode($this->encryption->encrypt($row->member_id)),
                    "profile_pic" => $row->profile_pic,
                    "name" => $row->first_name.' ' .$row->last_name,
                    "phone" => $row->phone,
                    "chss_no" => $row->chss_no,
                    "emp_id" => $row->emp_id,
                    "address" => $row->address
                    // "member_type" => $row->member_type
        );

         $array_secondary = array();
         $books_of_secondary = $this->Search_model->getTotalList_of_secondary($row->customer_id);
         foreach ($books_of_secondary as $key => $row) 
            {
                //print_r($row->customer_id);
                $arr_result2 = array(
                            "s_member_id" => base64_encode($this->encryption->encrypt($row->member_id)),
                            "s_profile_pic" => $row->profile_pic,
                            "s_name" => $row->first_name.' ' .$row->last_name,
                            "s_phone" => $row->phone,
                            "s_chss_no" => $row->chss_no,
                            "s_emp_id" => $row->emp_id,
                            "s_address" => $row->address
                );


                $array_secondary[] = $arr_result2;
            }
         $arr_result['secondary'] =  $array_secondary;  
         $data['data'][] = $arr_result;
      }
    echo json_encode($data);
    exit;
}

Y luego, en la función de formato, accedería al secondary campo para obtener una matriz de secundarios. Tal vez así:

function format(d) {
    // d is the original data object for the row var val;
    if(d.secondary.length == 0) {
        return "There are no secondary members";
    }

    var display = '<table cellpadding="5" cellspacing="0" border="0" style="padding-left:50px;">'; 
    for (val of d.secondary) { 
        display += '<tr>' + '<td>Full name:</td>' + '<td>' + val.s_name + '</td>' + '</tr>' + '<tr>' + '<td>Mobile number:</td>' + '<td>' + val.s_phone + '</td>' + '</tr>';
    }
    display += '</table>';
    return display;
}