Usando colunas fictícias para contabilizar as diferentes estruturas, uma união para juntá-las e uma seleção pai para lidar com a ordenação:
SELECT * FROM (
(SELECT foo.id, NULL AS title, NULL AS body, foo.downloads, foo.views, foo.created FROM foo)
UNION ALL
(SELECT NULL AS id, bar.title, bar.body, NULL AS downloads, NULL AS views, bar.created FROM bar)
) results
ORDER BY created ASC
