How to write sql subqueries

Can anyone help me out.
 
I want to select the "productid" for all products from a table called "products" that both "user1" and "user2" have purchased

I am using 

ALTER PROCEDURE test
(@user1 as int, @user2 as int)

AS

Select  productid from (Select productId
            From products
            Where productid=@user1)
           
WHERE productid=@user2

I am getting an incorrect syntax around the word "Where" message.

Can anyone tell me the correct syntax for this

Many thanks

Martin