Puoi sempre ORDER BY
in questo modo:
select
title,
( stock_one + stock_two ) as global_stock
from product
order by 2, 1
o avvolgilo in un altro SELECT:
SELECT *
from
(
select
title,
( stock_one + stock_two ) as global_stock
from product
) x
order by (case when global_stock = 0 then 1 else 0 end) desc, title