function get_category_tags($args) {
	global $wpdb;
	$tags = $wpdb->get_results
		("
            SELECT DISTINCT terms2.term_id as tag_id, terms2.name as tag_name
            FROM
                $wpdb->posts as p1
                LEFT JOIN $wpdb->term_relationships as r1 ON p1.ID = r1.object_ID
                LEFT JOIN $wpdb->term_taxonomy as t1 ON r1.term_taxonomy_id = t1.term_taxonomy_id
                LEFT JOIN $wpdb->terms as terms1 ON t1.term_id = terms1.term_id,
                $wpdb->posts as p2
                LEFT JOIN $wpdb->term_relationships as r2 ON p2.ID = r2.object_ID
                LEFT JOIN $wpdb->term_taxonomy as t2 ON r2.term_taxonomy_id = t2.term_taxonomy_id
                LEFT JOIN $wpdb->terms as terms2 ON t2.term_id = terms2.term_id
            WHERE
                t1.taxonomy = 'category' AND p1.post_status = 'publish' AND terms1.term_id IN (".$args['categories'].") AND
                t2.taxonomy = 'post_tag' AND p2.post_status = 'publish'
                AND p1.ID = p2.ID
            ORDER by tag_name
        ");
        $count = 0;
 
        if($tags) {
			foreach ($tags as $tag) {
				$mytag[$count] = get_term_by('id', $tag->tag_id, 'post_tag');
				$count++;
			}
        }
        else {
			$mytag = NULL;
        }       
        return $mytag;
}

<?php 
	$args = array( 'categories' => '1,2');
	$tags = get_category_tags($args);
	if(!empty($tags)) {
		foreach ($tags as $tag) {
			$content .= "<a href="".get_tag_link($tag->term_id)."">".$tag->name."</a>";
		}
	}
	echo $content;
?>