It’s difficult to say. In order to know why people can’t eat two Devil Fruits, we have to know the secret of Devil Fruits. We don’t know if immortality is the reason why people can’t eat two of them without dying. It’s confirmed that Teach has an unusual body, so having two Devil Fruits is most likely a physical, biological thing. I’ve heard a theory that states that eating a Devil Fruit replaces the person’s heart and that Teach gained the Tremor-Tremor Fruit by eating Whitebeard’s heart. If that’s true, then that explains why people can only eat one Devil Fruit.

My theory, however implausible it may sound, is every person with adequate will can consume multiple devil fruits. During the entire series, we saw exactly one instance of someone eating multiple fruits, and he succeeded. It could easily be another one of the government lies to limit the pirates strength.

Vegapunk said devil fruits were in fact, sort of human myths manifestation in inanimate objects, and awakening was the realization of the myth's potential. But one myth could possibly be both darkness and quake, for example.

Up to this date, we didn't see anybody else that have two distinct powers working for the government, none that we could guess might have eaten multiple devil fruits. And yet, something about this simple, strict rule feels off to me.

Good question won’t lie

@Zozo916 Vegapunk’s explanation of Devil Fruits is most likely a theory of his or a romanticized version of his own idea.They’re most likely an extremely advanced invention or a naturally occurring substance.

@Gfmorgan01 From what I remember, the elders made a comment on him being the first person in history to have more than one devil fruit. And this is coming from characters that are immortal and likely have been around since the Void Century.

@A Professional Idiot mk.2 I don't know where you're getting your info from but it was never said it was eternal youth, it was immortality.

Edit: Ok so you are right. It's called the immortal operation in the Funimation Dub which is why I got confused.

@Charys Da 2nd @A Professional Idiot mk.2

Technically, both of you are correct. From what we know so far, the Op-Op Fruit can grant immortality in the form of eternal youth, meaning the intended target doesn’t age. As for immortality, that’s a bit of a trickier matter. Usually, immortality means you can’t get sick, you don’t age, and you don’t die from normal causes. Since we don’t know how immortality works in the One Piece world, it could be the immortality or a tweaked version of immortality.

@Atlas D. Aeradon Haven't seen you in awhile

@Charys Da 2nd I’ve been on other wikis like Lord of the Rings, Naruto, Dragon Age, and the Last of Us.

Nice