today I would like to discuss matters relating to Blackbeard's ability.
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Simply put, my theory is that during Wano's arc we will know how Blackbeard is able to use the two forces of the devil fruit. I have been thinking about this since revealing that Marco is a doctor.
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Although this can always be inferred, based on the fruit of the devil is a phoenix, which is mythically associated with the nature of healing. But before this chapter is disclosed, we have never seen Marco explicitly referred to as a doctor, or providing medical care, or anything like that. Nor would it be too much for a Whitebeard's right hand to become a doctor. After all, in the One Piece world, Trafalgar Law is not just a pirate captain, but also a doctor.
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So if we make a logical guess that Marco not only became a doctor after Whitebeard's death, but he'd been a doctor before Whitebeard's death, then it would be a strong connection with one of the other lines related to Marco: what he knew about the body Blackbeard. According to Marco, there was something strange about Blackbeard's body, which could explain why he could use two devils at the same time.
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Of course his comments also imply that the whole crew knew the same info, so that does not mean Marco should be the one to find out, or the person who said it, but I assume that in any case he understands it better than any other Whitebeard crew. Being a doctor will give you insight on this. As a doctor, Marco can check Blackbeard when he was part of Whitebeard's crew, or maybe take care of him after an injury (perhaps after his bout with Shanks). With this opportunity and his medical knowledge, Marco might be able to discover something about Blackbeard's unique body that allows him to eat two different devils.