The way that I would do this is to create two separate tables (each one column wide) and then include both of them in a single, two-columned table:
<?php
$list=array('a','b','c','d','e','f');
$midpoint=floor(count($list)/2);
$tableHeader='<table width="100%">';
$tableFooter='</table>';
$leftTable=$tableHeader;
$rightTable=$tableHeader;
for ($c=0; $c<$midpoint; $c++)
{
$leftTable.='<tr><td width="100%">'.$list[$c].'</td></tr>';
}
$leftTable.=$tableFooter;
for ($c=$midpoint; $c<count($list); $c++)
{
$rightTable.='<tr><td width="100%">'.$list[$c].'</td></tr>';
}
$rightTable.=$tableFooter;
$mainTable='<table><tr><td width="50%">'.$leftTable.'</td><td width="50%">'.$rightTable.'</td></tr></table>';
echo $mainTable;
?>
Add some CSS to remove padding around the sub-tables and borders etc and you should be good to go.