Re-read the Ace Novel.
Blackbeard's character is pretty interesting.
One thing Oda said regarding Blackbeard is that if we listen to what Ace said about him back on Banaro Island. Ace said that Blackbeard had been on the ship twice the length of others.
If we take into account that Luffy said he's multiple people, we can assume or come to the conclusion that Blackbeard is 2 people.
Blackbeard also doesn't sleep; if people stay up for 12 hours and sleep the other 12 hours, the ability to stay awake would mean they're experiencing life twice the rate of normal people.
I think maybe the reason Blackbeard is special is because he's actually got 2 organs:
If we assume he's basically got a factor of x2, then maybe he's 2x as strong as he'd otherwise be... or why he seems to exaggerate pain because it feels like he's feeling the pain of 2 people. Could explain why he's so strong. Maybe he's strong because he literally has the power of 2 people. 2 hearts could explain why he has 2 DFs.
Other reason I think he's got 2 and not 3 (which is what most people think) is Oda said Ace's statement of "twice" was the hint. It explains why he only has 2 Devil Fruit powers.
If he were capable of possessing 3 abilities, I'd imagine he'd have 3 abilities by now. Instead of having Burgess acquire the Mera Mera no Mi for himself, he'd likely have gone to Dressrosa himself to take the Mera Mera no Mi for himself, so I am personally sticking with he's got the organs (2 hearts at list) of 2 people
