Ok, that’s different. Don’t use the ‘Free products’-option but instead:

– Discount type: Percent discount
– Coupon amount: 100%
– Usage restriction – Minimum quantity of matching products: 2
– Usage limits – Limit discount to: Every nth item

This will make every 2nd item in the cart free.

If you want to limit the amount of free products to 1, choose ‘Lowest priced product’ instead.