Jag tror att du vill:
select customer,
sum(case when area = 'X' then 1 else 0 end) as x,
sum(case when area = 'Y' then 1 else 0 end) as y
from t1
where exists (select 1
from t1 tt1
where tt1.customer = t1.customer and tt1.product = 'A'
)
group by customer;