very rough edited version here probably with some errors and maybe tags not all closed -
also it will do too many nested tables but it should help
set a counter $j
set a stopvalue - you could define it as half your returned count - the one here is a hardcoded 6 - when it reaches that value it does '</td><td>spacer</td><td>' and then carries on
<table>
<tr>
<td>
<?php
$j=0;
$result = mysql_query("SELECT * FROM $revutbl_name, members WHERE $revutbl_name.mem_id=members.mem_id AND pare_id=0 ORDER BY dat desc LIMIT 12 ", $db) or $error.=(mysql_error());
while ($row=mysql_fetch_array($result))
{
$thisrevu_id=$row["revu_id"];
$thispare_id=$row["pare_id"];
if ($j==6) echo '</td><td> </td><td>';
?>
<table border='0' cellspacing='0' cellpadding='0' width='345'>
<tr>
<td rowspan='3' class='onav3'><img src="images/spacer.gif" height='5' width='2'></td>
<td colspan='2' ><?php echo $row['hdng'];?></td>
<td align='right' class="<?php echo $headcolr ;?>"><a href="<?php echo $PHP_SELF,'?pgtype=wrty&pare_id=',$thispare_id;?>" class="revu">Reply</a> </td>
<td rowspan='3'><img src="images/spacer.gif" height='5' width='2'></td>
</tr>
<tr>
<td ><?php echo $row['mem_username'];?></td>
<td ><?php echo $row['date']; ?></td>
<td><?php echo $row['rating'];?> </td>
</tr>
<tr>
<td colspan='3' ><?php echo $row['txt']; ?></td>
</tr>
</table>
<?php
$j++;
}
?>
</td>
</tr>
</table>