I create a table with 2 lines and 2 columns. Then I merge the 1st cell with the cell below; I make the same with the 2nd cell. Then I have 1 line with 2 columns :
<tr>
<td rowspan="2">...</td>
<td rowspan="2">...</td>
</tr>

Then....I can't split the cells and I can't add a line....