well..I'm kind of a detective..and I think I could guess the reason for this. its somewhat explained in parts by the other users in this post. This is how it goes.: Blackbeard has no abnormal body, its just normal like others. It was his plan all along. he knew that he alone was no match to fight others, hence he joined the whitebeards Then he studied the various Fruits and their abilities, then can across his Yami Yami no mi fruit. he learned that the fruit could nullify/block any other fruit's power. This was his masterplan. And he killed the other commander just to get his hands on the fruit. He trained himself with using the fruit while he was running away from ACE who was searching him Now comes the most important part. It was clearly explained in the Ennies Lobby parts, that there have been several attempts by others to try and eat two devil fruits, but they died as the two devil interacted with each other in his body. Also there is another side story that states, "When a fruit user dies, the powers that he acquired from his body are lost and return in the form of a fruit." So I guess BlackBeard knew this, and after WhiteBeard (R.I.P.) died, he acquired the fruit easily. Now about the two devils interacting, I think that his fruit is not a devil but actually a shield. And when he uses the other fruit he makes a weird shield in his body, thus safe-guarding him as well as allowing him to use the respective abilities of the fruit. Lol, mine is also a theory, but I think this would quite suite the situation. Anyone is welcome to find errors in it.