This is an HTML design problem, not an XSLT problem. What you want is "nested tables". Do some googling to learn how to nest 2 tables within a second table. Here's the first link I found.
http://www.htmlcodetutorial.com/help/archive.php/o_t__t_1472__i-need-tables-side-by-side..html
And example would look something like this, but there's many different ways to do it.
<table width="100%" border="1" cellpadding="5" cellspacing="0" bordercolor="#FF0000">
<tr>
<td><table width="100%" border="1" cellpadding="5" cellspacing="0" bordercolor="#00FF00">
<tr>
<td>table in left cell </td>
<td>and another cell </td>
</tr>
</table></td>
<td><table width="100%" border="1" cellpadding="5" cellspacing="0" bordercolor="#0000FF">
<tr>
<td>Table in right cell </td>
</tr>
<tr>
<td> </td>
</tr>
</table></td>
</tr>
</table>
Once you know what your HTML node tree needs to look like, creating that in XSLT is just like creating any other node tree structure.