How to have the cart discount when 2 articles purchased?

Hi,

I have a special offer if customers buy 2 items ( they all have the same price ). So if the customer put 1 item in the cart the normal price is counted but if he adds another one ( so for 2 items but the offer is only valid for 2 items ) the total price is discounted by 25%.
How can we do that?
Best regards and all my thanks in advance,

Thierry