To check for customers who have ordered both - ProductID 2 and 3, HAVING can be used
select customerId from orders where productID in (2,3) group by customerId having count(distinct productID) = 2
The query selects only records with the productIDs in questions and with the HAVING clause checks for groups having 2 productIds and not just one.
Another possibility would be
select customerId from orders group by customerId having sum(case when productID = 2 then 1 else 0 end) > 0 and sum(case when productID = 3 then 1 else 0 end) > 0
This query selects only groups having at least one record with productID 2 and at least one with productID 3.