Create 2 items for a set price

I am trying to create a promotion that will work when a customer buys 2 items together. For example: the promotion will be 2 items for $20, but if a customer added a 3rd item to the cart it would only discount the first 2 items and the 3rd item would be full price, unless the customer order 4 items, then the discount would apply to all 4, so basically the discount should only apply if there is an even number of items in the cart.