Try something like:
SELECT ui.user_id,
SUM(CASE WHEN date_part('year', o.time_stamp) = 2012 AND date_part('month', o.time_stamp) = 07 THEN o.amount ELSE 0 END) jul_12,
SUM(CASE WHEN date_part('year', o.time_stamp) = 2012 AND date_part('month', o.time_stamp) = 08 THEN o.amount ELSE 0 END) aug_12,
FROM orders o JOIN users_info ui ON ui.user_id = o.user_id
WHERE user_id = '123456'
GROUP BY ui.user_id;
You were getting one row per order because you were grouping by o.time_stamp
and timestamps are different for each order.
A shorter version of query:
SELECT ui.user_id,
SUM(CASE WHEN date_trunc('month', o.time_stamp) = to_date('2012 07','YYYY MM') THEN o.amount END) jul_12,
SUM(CASE WHEN date_trunc('month', o.time_stamp) = to_date('2012 08','YYYY MM') THEN o.amount END) aug_12,
FROM orders o
JOIN users_info ui ON ui.user_id = o.user_id
WHERE ui.user_id = '123456'
GROUP BY ui.user_id;