I've been watching for the 2 20lb packs for $10 for a long time. I haven't seen it in 2 or 3 years now. They still sell the twin-packs. But, they reduced the weight and kept the price the same. If my math is correct, $5 for 15.6 lb is 28% higher than 40 lb (twin-pack of 20's) for $10.
Since they've raised prices, I've been experimenting with other brands. Along with experimenting with these other brands and my stockpile of Kingsford from the last good sale, I haven't had to buy Kingsford in several years and still have probably 10 bags.