He asked Oda to have 2, and Oda said “well yeah why not?” And gave him 2

My first guess is that he is a cyborg, and probably fed his robot half one fruit, while his human half posesses another. My other guess is that either the yami or gura is artificial, and perhaps one can posess one real and one artificial devil fruit.

I personally think it either has something to do with the Dark-Dark Fruit properties, or Teach is... probably not a human. Don't forget that the guy never sleeps.

He has three devil fruits and that the real Blackbeard has a devil fruit which allows him to clone himself and his two clones have devil fruits. The flag on Blackbeard's ship also has three skulls.

Marco said that he has unique biology. Which Marco wouldn’t know if it was because of the Yami Yami fruit, so it’s unrelated to that

My guess is that it has something to do with his weird biology. On Marineford Marco speculates that his body is “strange” so maybe that’s the reason. Additionally, when Zoro and Luffy first meet Blackbeard, they mentioned that Blackbeard might be two or more people at once somehow. That’s kind of the best evidence I have to go off of.

His flag also has three skulls

And his has three complete different personalities

When we are showed what happens when you consume a devil fruit it seems to bond to your soul that's why you lose it when you die so it would make sense that blackbeard had big mom split his soul allowing him to have multiple devil fuits

Monstertoots I like that theory a lot. I hope it’s real