If you need to have your categories display in two columns all you need to do is modify the the stadard wp_list_categories() functions to look like the code below

<?php
$cats = explode("<br />",wp_list_categories('title_li=&echo=0&depth=1&style=none'));
$cat_n = count($cats) - 1;
for ($i=0;$i<$cat_n;$i++):
if ($i<$cat_n/2):
$cat_left = $cat_left.'<li>'.$cats[$i].'</li>';
elseif ($i>=$cat_n/2):
$cat_right = $cat_right.'<li>'.$cats[$i].'</li>';
endif;
endfor;
?>
<ul class="left">
<?php echo $cat_left;?>
</ul>
<ul class="right">
<?php echo $cat_right;?>
</ul>

Hat Tip: WP Recipes.com