Group And Sort After Union

Greetings!

I want to GROUP BY and ORDER BY on the result of UNION similar to the
following (wrong) SQL:

(SELECT c11 AS c1,SUM(c12) AS c2 FROM table1
UNION
SELECT c21 AS c1,SUM(c22) AS c2 FROM table2
)
GROUP BY c1
ORDER BY c2;

Please note that the following is NOT what I want because it generates
2 groups of data set:

SELECT c11 AS c1,SUM(c12) AS c2 FROM table1
GROUP BY c1
ORDER BY c2
UNION
SELECT c21 AS c1,SUM(c22) AS c2 FROM table2
GROUP BY c1
ORDER BY c2;

How do I do that? Thank you in advance!

CNLIOU