To add 2 or more product to an order in the ADD NEW ORDER, in part 2 of the page, search for the product again in the Search box.  Then select the product and it will open up a new Product box to complete.  When finished press ADD PRODUCT and this will add the additional product.

If you are trying to add the same product twice because of a different date or because of a variance in the product, search for the same product in the search box and select the same product.  This will open up the product box again for you to add the product.  When finished press ADD PRODUCT and this will add the additional product.