Display posts matching two categories.

I need to select posts that belong in TWO categories.
How do I add “category__and” into this code?
Please help!

function excerptlist_func($atts) {
	extract(shortcode_atts(array(
		'titleonly' => 'n',
		'cat' => '',
		'num' => 5,
		'offset' => '',
	), $atts));

	$args = array('posts_per_page' => $num, 'offset' => $offset );
	$id = get_cat_id($cat);
	$args['cat'] = $id;

	$loop = new WP_Query( $args );
        $posts = array();
        foreach($loop->posts as $p) { $posts[] = $p; }
	global $post;
	$opost = $post;
	ob_start();