getting posts from two categories

Hi

I have a small blog. I want to show posts from categories that have the numbers 10 and 15 – and order them by date

I have tried this:

<?php
   if (is_home()) {
query_posts( array ( 'category_and' => array(10,15), 'posts_per_page' => 20, 'orderby'=>'date', 'order'=>'DESC' ) );
}
?>

but it doesn’t work – it gives me 20 posts, but not restriced to the two categories

Please can someone help.