Like this http://jsfiddle.net/8ha9e/1/
Add colspan="2"
to the 3rd <th>
and then have 4 <td>
‘s in your second row.
<table border="1">
<tr>
<th scope="col">Header</th>
<th scope="col">Header</th>
<th scope="col" colspan="2">Header</th>
</tr>
<tr>
<th scope="row"> </th>
<td> </td>
<!-- The following two cells will appear under the same header -->
<td>Col 1</td>
<td>Col 2</td>
</tr>
</table>