Equivalent SQL exitsts translation in MDX

Hello,
I've a problem wirting a MDX statement. The statement should sum up the turnover for a customer only if the order contains at least two items with different promotion types.

In SQL, this statement is rather easy. Using the Adventure Works DW database the SQL statement would be something like

select f1.customerkey, sum(extendedamount)
from factinternetsales f1
where exists (
select *
from factinternetsales f2
where f1.salesordernumber = f2.salesordernumber
and f1.promotionkey <> f2.promotionkey
)
group by f1.customerkey

But how to tanslate the exists statement in MDX using the Adventure Works cube Please let me know any suggestions.
thanks