Blackbeard is by far one of the most intriguing character in One Piece, from his background to his insatiable quest for power. From what we know so far about him, he joined the WB pirate in search of the yami yami no mi. After finding it, he set his final plans in motion: become a shichibukai to gain access to impel down, and leave with the most fearsome and powerful pirates to form the hulk of the Blackbeard pirate. Then kill whitebeard and obtain the gura gura no mi.
Now I'm stuck deciding if Blackbeard was confident that he could take WB down at any time after obtaining the fruit and forming his crew. But that's a question for another time.
The question now is if it's at all possible that the yami yami no mi fruit gives its users the ability to assimilate another devil fruit inside his body, considering the darkness is the "ability to pull everything in" coupled with blackbeard being referred to as "them" rather than a single person. I'm trying to make a connection with darkness itself. The fact that the yami yami no mi fruit is characterized as the most evil, maybe that's somewhat of a prerequisite to withstand another devil, kinda like trading the yin-yang, for a full yin.
I'm basically hinting at the fact that BB knew exactly which two fruits he wanted after researching them. The fact that he knew that he could get two while everyone else can only get one is absolutely nuts. I think the key to understanding his confidence is in the yami yami no mi... Or I'm just crazy