If I understand correctly you want to set it up so depending on the order total a different free item is discounted?
If so you can do this with a parent coupon:
Coupon A: buy x get y, minimum value 10, free item for 10
Coupon B: buy x get y, minumum value 50, free item for 50
Coupon C: parent coupon, add coupon A and B, where B is ordered before coupon A and process type of first found match
Now if you set the 'automatically add to cart', then it will add the free product to the cart, but it does not remove it later if the customer does not qualify for the discount. So there could be a scenario where the customer has both free items in the cart, but only getting a discount for one.