Build an array of categories:
foreach ($categories as $category) {
$cat[] = $category->cat_ID;
}
Then pass that to the query:
$args=array(
'category__in' => $cat,
'order' =>DESC,
'post__not_in' => array($post->ID),
'posts_per_page'=>2,
);
In other words, put the query after the foreach
, not inside of it.