Discount for each additional Item
You could potentially add a "data comparison" condition to your existing discount rule that would effectively do this:

current-user:order:line-items:0:commerce_product:id != product:id

Which would say "only continue if the first line item's product id isn't the same as this product id" but would totally fail if you had some other line item added to your cart before your first product. If that is the case, then you would need to create a custom rules condition.