Average values from different table on join

CREATE TABLE `reviews` (
  `id` int(11) NOT NULL,
  `average` decimal(11,2) NOT NULL,
  `house_id` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `reviews` (`id`, `average`, `house_id`) VALUES
(1, '10.00', 1),
(2, '10.00', 1);
ALTER TABLE `reviews`
  ADD PRIMARY KEY (`id`);
ALTER TABLE `reviews`
  MODIFY `id` int(11) NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=3;

CREATE TABLE `dummy_reviews` (
  `id` int(11) NOT NULL,
  `average` decimal(11,2) NOT NULL,
  `house_id` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `dummy_reviews` (`id`, `average`, `house_id`) VALUES
(0, '2.00', 1);
ALTER TABLE `dummy_reviews`
  ADD PRIMARY KEY (`id`);

AND the query

SELECT
  AVG(r.average) AS avg1,
  AVG(dr.average) AS avg2
FROM
  reviews r
LEFT JOIN
  dummy_reviews dr ON r.house_id = dr.house_id

the result is

avg1        avg2    
10.000000   2.000000

All good by now but (10 + 2) / 2 = 6 ... wrong result

I need (10+10+2) / 3 = 7,33 ... How can I get this result?

SQLFiddle