MySQL query optimization with aggregate ORDER BY
Did you tried in the following way:

SELECT * FROM (
SELECT i_id, n_id, SUM(m1) AS sum1, SUM(m2) sum2
FROM t1
WHERE b_id=1234 AND date&gt;='2013-06-01' AND date&lt;='2013-08-31'
GROUP BY i_id, n_id
LIMIT 10 OFFSET 0)x
ORDER BY sum1 + sum2 DESC


I'm in SQL Server and there it's helping me.