Seleccione filas separadas de dos tablas, ordene por fecha

Usando columnas ficticias para dar cuenta de las diferentes estructuras, una unión para unirlas y una selección principal para manejar el orden:

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