Black Beard's Devil fruit [theory]

It has been circulated over the web about blackbeard's devil fruit which is the "cerberus threory" and is said to be the most accurate from any other theories out there but Obviously, that theory was only based on black beard's "jolly roger" which resembles the appearance of a "cerberus" and also the scratch of Shank's left eye.
However I don't consider this to be legit or I could say , this is far from being real .

first , I will show you how shanks got that scar on his left eye .

 It has been explained on the theory mentioned above that shank's got the scar on his left eye from the claw of cerberus but take a look at this picture. 

So, how did blackbeard got a second devil fruit ? It would be impossible for a human to acquire a second fruit but as what the first division commander Marco said , blackbeard is a different being , and that is also the reason why his body can endure the side effects of "yami yami no mi" which is to double the pain acquired from any attacks.
And my theory is , Black beard's body has the ability to acquire a second fruit.
Marco already knew about black beard being different from the others since they're both crew mates for several years and also he already seen black beard's skills many times, that's why he's not surprised when he saw black beard using their captain's power.