If your products don't have options = like colors, a really easy, fast way is to use the promotions tab on the product. If you have options, they can only order the same color (option) .
Simply set the qty to 2, and use half the price. if customer orders only one it is the full price, if they order 2 it is still just the full price. eg one is 25. 2 will still be 25. You need to set maximum qty to 2, or if they order 3 they will get the 12.50 x 3 unless you want customer to be able to buy 3 at reduced price.
Add a blub about the BOGO - see attachment
