As documented in the codex you could use the following
$query = new WP_Query( array( 'category__and' => array( 2, 6 ) ) );
//post has to be in category with ID 2 AND 6
As documented in the codex you could use the following
$query = new WP_Query( array( 'category__and' => array( 2, 6 ) ) );
//post has to be in category with ID 2 AND 6