Dreadmaster's original attack points is on the card, which is "?." Because it is not defined specifically, the default attack is treated as 0.
Dreadmaster's ATK can only be increased by the same amount of the ORIGINAL attack, which is considered 0. So if you have two Dreadmasters, they gain 0 ATK from each other.
In this case, Hannibal is correct. If a Destiny Hero such as Captain Tenacious is summoned, both Dreadmasters gain 800 Attack Points due to the ATK of the Tenacious. They do not take ATK Points off each other.