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.
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