There are theories as to why Blackbeard is able to have multiple devil fruits. Such as him having a Cerberus type of physiology (devil fruit) but I have a more sinister take on things. I don’t know if anyone has already said this, since I haven’t heard anything said of it yet, but what I’m about to suggest might paint Blackbeard in an even darker light then the Yami Yami no mi so here it goes…. I believe that the reason Blackbeard can have multiple devil fruits is because of the conditions of his birth… In our world there are mysterious cases of disappearing twins in the womb, later it was found out that one twin absorbed the other or a partial absorption took place and the twins were physically joined together. There have been many real life occurrences of this happening along with a media following in movies and tv shows.
What I’m proposing is that its fine for Blackbeard to have multiple devil fruits without destroying himself because he is not one person but two. The one devil fruit he possesses is the Yami Yami no Mi while the Gura Gura no Mi is possessed by a twin that resides in Blackbeard’s body. This is why even though Blackbeard possesses 2 devil fruits it still does not destroy his body as accounts say should happen. To support this it was stated by Marco, Blackbeard’s former crew member, that Blackbeard possesses an “atypical body structure” which is why he can have 2 devil fruits. But saying this is a HUGE hint when taking into account that One Piece is filled with all types of characters that you can easily call atypical, like people as tall as mountains (but not giants), people with long arms and legs (like the tribes of the same names), people with three eyes and so on… to put it simply-there are more atypical people then normal ones in the story line (just think about all the characters that the straw hats have come across). To call Blackbeard atypical would mean that he is an oddity even among all these oddballs we have come across so far and I’m not saying that lightly. This is because Oda is a master of leaving hints here and there and he does not set time bombs like Marco’s statement as a throwaway line. This has been the same when he foreshadowed so many other events while given small hints hundreds of chapters before (like haki for example). And in my perspective having an inborn twin that Blackbeard assimilated into his body at birth not only justifies the “atypical body” statement but the ability to have 2 devil fruits too. And who else but Marco would know this because he was on the same crew as Blackbeard before Blackbeard’s treachery. By the translation I’m going with Marco says “It would be impossible for a normal human. But you should all know that Teach is a little different!! The very structure of his body… Is strange!! Is that how he pulled this off?!” This statement was said to the whitebeard crew because they were the people closest to Blackbeard in the past and they would know that the strange part about him was that he had a deformity that was very odd even in the world of One Piece. They knew this and that is why those closes to him were able to explain how he did it and not anyone else from any other crew or the Marines.
My second point is the use of the Yami Yami no mi. It is never explicitly shown the methods Blackbeard used to harvest Whitebeard’s fruit but knowing that when a user dies the fruit is reborn close to the dead user’s body I’ll make an inference from that.
Last I wanted to comment how I’ve seen how devil fruits both shape a person’s ‘self’ and how that person changes their fruit (by using it through their own creativity). Like Whitebeard, a stoic badass that towers over people with huge strength, has an earthquake fruit that makes the ground tremble underfoot. Luffy, so blunt that common sense bounces off of him, but he is flexible as a character and makes friends out of his enemies (Nico Robin for one). Luffy changed his rubber power into a powerhouse because he needed the strength to protect his friends (all the gears). Then there is Blackbeard who possesses the Yami Yami no mi, in one word, darkness. We first learn that he is a runaway form Ace because Blackbeard killed his own crew mate to take the Yami Yami no Mi. Blackbeard then captures Ace which leads to Ace’s death… Everything this man does is dark which matches his fruit perfectly (even though he purposefully looked for that fruit). In that darkness Blackbeard keeps his secrets. Something so dark but so fitting. That Blackbeard has an ingrown twin…
*Theory by Verdeofgreed