That can get a little tricky. I'll need a little further clarification on how you want this to work.
Should they only get one for free? or for each one they purchase, should they get one for free.
In other words, if they order 4 of a product the discount applies to, should the discount be for 2 free or only 1 free and the other 3 at regular price?
where the discount applies to multiple products, do you want it to work that if they order 1 each of 2 products the discount applies to, they should get one of them free? is the discount applied to the aggregate of products it applies to or only at a product by product basis?