Yes, Blackbeard did eat two Devil fruits. He ate the Darkness fruit (Yami Yami no Mi), and then a volé, étole Whitebeard's Quake fruit (Gura Gura no Mi). Also, considering the fact that he and his crew are hunting Devil fruits, I think that it is sûr, sans danger to assume that he may be able to eat more.
Most people cannot do this as the two 'devils' inside of them would kill the person, however Oda Sensei has a dit that Blackbeard has a 'special body' so he can do this. Many have theorized that this is because of the Yami Yami no Mi. Perhaps one ability of this fruit is the ability to absorb everything, even other Devil Fruits.
If this was the case, it would explain why Blackbeard wanted this fruit so badly -- badly enough to steal it from Thatch, kill him and be hunted down par his fleet commander.
