mu-plugin1.php

<?php

/*
 * Show posts assigned to 'International (intl)' Edition
 * under all editions.
 *
 * http://wordpress.org/support/topic/show-posts-from-multiple-categories-1?replies=3#post-4910968
 */
function wpse10691_better_editions( $query ) {

	$get_edition = get_query_var( 'category_name' );
	if( $get_edition !== '' ) {
		$edition_obj = get_term_by( 'slug', $get_edition, 'category' );
		if( $edition_obj && !is_wp_error( $edition_obj ) ) {
			$edition = $get_edition;
		}
	}

	// Modify main query
	if( $query->is_category() && isset($edition) && $query->is_main_query() ) {

		$query->set( 'category_name', $edition.', intl' );

	}

}
add_action( 'pre_get_posts', 'wpse10691_better_editions' );

?>