合并GridView的表头

合并GridView的表头：

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class _Default : System.Web.UI.Page 

<?xml version="1.0" encoding="utf-8" ?>
<EmployeeDetails>
  <Employee>
    <Department>Development</Department>
    <DepartID>2</DepartID>
    <Name>Rajendran</Name>
    <Age>25</Age>
    <Location>USA</Location>
  </Employee>
  <Employee>
    <Department>Development</Department>
    <DepartID>2</DepartID>
    <Name>Karthic</Name>
    <Age>25</Age>
    <Location>USA</Location>
  </Employee>
  <Employee>
    <Department>Testing</Department>
    <DepartID>2</DepartID>
    <Name>Karthikeyan</Name>
    <Age>25</Age>
    <Location>India</Location>
  </Employee>
  <Employee>
    <Department>Management</Department>
    <DepartID>4</DepartID>
    <Name>Vidhya</Name>
    <Age>24</Age>
    <Location>India</Location>
  </Employee>
  <Employee>
    <Department>School</Department>
    <DepartID>-1</DepartID>
    <Name>Anandh</Name>
    <Age>24</Age>
    <Location>India</Location>
  </Employee>
  <Employee>
    <Department>Mechanic</Department>
    <DepartID>5</DepartID>
    <Name>Magesh</Name>
    <Age>25</Age>
    <Location>India</Location>
  </Employee>
  <Employee>
    <Department>Admin</Department>
    <DepartID>7</DepartID>
    <Name>Sabari</Name>
    <Age>25</Age>
    <Location>India</Location>
  </Employee>
  <Employee>
    <Department>Human Resource</Department>
    <DepartID>8</DepartID>
    <Name>Nirmal</Name>
    <Age>25</Age>
    <Location>India</Location>
  </Employee>
</EmployeeDetails>