select *
from user
where id in (select user_id from order where syouhin_id=A)
and id in (select user_id from order where syouhin_id=B)

簡単な記述で応用の効く例

SELECT a.*
FROM `user` a
WHERE EXISTS(SELECT * FROM `order` b WHERE b.user_id = a.id AND b.syouhin_id = A LIMIT 1)
 AND EXISTS(SELECT * FROM `order` b WHERE b.user_id = a.id AND b.syouhin_id = B LIMIT 1)
;

動作速度面で少し有利な例

SELECT a.*
FROM `user` a
 LEFT JOIN `order` b1 ON b1.user_id = a.id AND b1.syouhin_id = A
 LEFT JOIN `order` b2 ON b2.user_id = a.id AND b2.syouhin_id = B
WHERE b1.order_id IS NOT NULL
 AND b2.order_id IS NOT NULL
;

お暇な時に以下参照
http://dev.mysql.com/doc/refman/5.1/ja/optimizing-subqueries.html

SELECT * FROM user
WHERE id in (SELECT user_id FROM ORDER WHERE syouhin_id=A)
AND id IN (SELECT user_id FROM ORDER WHERE syouhin_id=B);