As the Wiki is mainly driven by Markdown, If you want to do anything it should be done in the markdown. So the below workaround would be the only way to do this.
Workaround.
Use HTML tables on Azure devops Wiki:
<table>
<tr>
<td>H1</td>
<td>H2</td>
</tr>
<tr>
<td colspan="2">content</td>
</tr>
</table>
I hope this helps you.