Combining Category Arrays with array_merge

Hi,

I’m developing a custom category page template that’s trying to merge two arrays retrieved by get_categories with the array_merge function.

While it’s retrieving the results, and I’m able to output the category data in a foreach loop. For some reason, the categories are being outputted in a seemingly unpredictable order; neither in an ascending or descending order as defined in my arguments. Can anyone point me to where I’m going wrong here?

Basically, I would like to retrieve posts from two categories and their children in a specified ascending order. Any advice would be greatly appreciated.

<?php 	function portfolio_browser(){    

			global $cat;

$arg1 = array(
'type' => 'post',
'child_of' => 36,
'order' => 'DESC',
'order_by' => 'cat_ID',
'hide_empty' => false,
'hierarchical' => true
 );

$arg2 = array(
'type'=> 'post',
'child_of'=> 23,
'order_by' => 'cat_ID',
'order'=> 'DESC',
'hide_empty' => false,
'hierarchical'=> true,
'exclude' => array(27, 28, 29, 31, 32)
 );

 $projectcats = get_categories($arg1);
 $teachingcats = get_categories($arg2);
 $combinedcats = array_merge($projectcats,$teachingcats);				 ?>

<?php foreach ($combinedcats as $combinedcat): 											           $slug = $combinedcat->slug;
 $cat = $combinedcat->term_id;?>
																		<?php if ($cat == '33') { ?>

										<section id="teaching">
												<h1>Teaching</h1>
												<div class="content-item text">

														<?php 	$key="Teaching Text";
																$key2="Teaching Image"; ?>

														<img src="<?php echo get_post_meta($post->ID, $key2, true);?>" alt="Teaching Section Header Image"> 

														<p><?php echo get_post_meta($post->ID, $key, true); ?></p>

												</div>

										</section><!--#teaching-->

									<?php } ?>	            	

<section id="<?php echo $slug ?>" class="content-item">

	<?php bigwitz_portfolio_view(); ?>

</section><!--<?php echo $slug ?>-->
			<?php  endforeach;  

		} ?>