如何将表的前两列堆叠成一列？

<table>
  <tr>
    <th>列1</th>
    <th>列2</th>
    <th>列3</th>
  </tr>
  <tr>
    <td>数据1</td>
    <td>数据2</td>
    <td>数据3</td>
  </tr>
  <tr>
    <td>数据4</td>
    <td>数据5</td>
    <td>数据6</td>
  </tr>
</table>

table {
  display: flex;
  flex-direction: column;
}

th, td {
  flex-basis: 50%;
  text-align: center;
}