SELECT *
FROM
(
    SELECT case_id,diagnosis_title,updated
    FROM tbl_case
    order by updated desc
) as table1

UNION

select *
FROM
(
    select image_id,image_title,updated
    from tbl_image
    order by updated desc
) as table2

(SELECT case_id, diagnosis_title, ... FROM ...)
UNION
(SELECT image_id, image_title, ... FROM ...)
ORDER BY ...

SELECT case_id,diagnosis_title,updated
    FROM tbl_case
    UNION
    select image_id,image_title,updated
    from tbl_image
    ORDER BY updated desc

CREATE TEMPORARY TABLE TempTable
    (id int, title varchar(100), updated tinyint(1));

INSERT TempTable
SELECT case_id, diagnosis_title, updated  FROM tbl_case ORDER BY updated DESC;
INSERT TempTable
SELECT image_id, image_title, updated FROM tbl_image ORDER BY updated DESC;

SELECT * FROM TempTable;

SELECT -1 as 'id' , 99999999999999 as 'sortalias' , now() as 'datum||date'
 UNION
 ( SELECT
      id
      ,unix_timestamp(datum) as 'sortalias'
      ,datum as 'datum||date'
      FROM kunde
 )
 order by sortalias desc