Only showing posts that is in both in categories X & Y

Hi

I am currently trying to work out the best way to only show posts which are in 2 specific categories, so post’s that are in category X and in category Y. I have currently done it by the below code but it isn’t an great solution since I will end up with less posts than what my limit is set at.

query_posts( 'posts_per_page=10&cat=10,15' );                

 while ( have_posts() ) : the_post();

     if ( in_category(15) &&  in_category(10)) {

?><li><a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title_attribute(); ?>"> <?php the_title(); ?></a></li><? 

     } 

endwhile;

wp_reset_query();

Anyone got any suggestions? or How would I manage it via a MySQL query?