Why one horse can pull six tons and 2 horses can pull 36 tons?

There is no real explanation for this, but two horses harnessed together will always be able to pull more weight. Typically a draft horse can pull it's own weight and up to twice it's own weight on it's own. So for a 2000 pound draft that's 2000 to 4000 pounds that it can pull on it's own. Harness two horses together and they can pull 4,000 to 8,000 pounds.