In some English accents, when you say "two for" quickly, it sounds like "two fer". Two-for-one is a way of saying that if you buy one, you also get one for free. So the phrase "two-fer" often implies a two-for-one offer.
Imagine a bakery that has a holiday offer where you can buy two cookies for the price of one ("two-fer one!"). You go for the offer and (very generously) decide to give the extra cookie to a friend.