gistfile1.php

<?php
add_filter( 'pre_get_posts', 'cambia_queries', 10, 1 );
function cambia_queries( $query ) {

	if ( $query->is_category( 31 ) && $query->is_main_query() ) {
		$query->set( 'cat', array( 18, 31 ) );
		$query->set( 'posts_per_page', - 1 );
	}

	return $query;
}