Few One Piece characters are as vile as Marshall D. Teach, or Captain Blackbeard. Starting out as a member of Whitebeard's crew, Teach betrayed his surrogate father and siblings when he killed Thatch to steal the powerful Dark-Dark Fruit, a Logia type. Years later, Blackbeard captured Portgas D. Ace and handed him to the World Government to gain the status of a Warlord. Blackbeard did all this so he could cause a war that would allow him to acquire the crew he wanted, kill Whitebeard and steal Whitebeard's own Devil Fruit powers, but how can Blackbeard have two Devil Fruits?
Curiously, Blackbeard is the first human in One Piece history capable of wielding two Devil Fruit powers at once, supposedly an impossible feat. CP9 agent Jabra himself reveals this is the case, proving this limitation wasn't some elaborate World Government conspiracy. Additionally, both the Marines and the pirates were shocked after seeing Teach wield two separate powers, and neither they nor One Piece fans know exactly how this happened. How does Blackbeard have two Devil Fruits? A variety of One Piece fan theories offer some intriguing possibilities.
The Cerberus Theory
Blackbeard already ate a Devil Fruit.
The Cerberus Theory, easily one of the most popular within the One Piece community, posits that Blackbeard had already eaten a Devil Fruit before he ate the Dark-Dark Fruit. Specifically, a Mythical Zoan Fruit possibly called the Dog-Dog Fruit, Model Cerberus. Theorists believe that, like the mythical Cerberus' three heads with independent thought, Teach is able to use more than one Devil Fruit. The justification for this theory mainly comes from how Blackbeard and his actions tend to revolve around the number three.
The first of these is Blackbeard's Jolly Roger, meant to represent a ship's captain. His specific flag features the standard skull and crossbones, but curiously has three skulls that are quite close together -- like that of a Cerberus. Then there is his supposed weapon of choice, a three-bladed claw he's used since he first joined the Whitebeard Pirates. This claw is actually responsible for Shanks' famous scars across his left eye -- or it could have been the claw of Blackbeard's hypothetical Zoan form. The fact that the myth of Cerberus exists in One Piece's world gives this theory even more credibility. As for "how can Blackbeard have 2 Devil Fruits?", this theory hints that two is the wrong number, and Captain Blackbeard may have room for a third Devil Fruit.
Chimera Theory
There are multiple entities inside Blackbeard.
How does Blackbeard have 2 Devil Fruits? Much like the Cerberus Theory, the Chimera Theory aims to answer this question with another "multiple entities inside Blackbeard" approach. While the former implies they come from eating a Devil Fruit, the Chimera Theory states that this is just how Blackbeard was born. Given how Marco the Phoenix even said during Marineford that Blackbeard's body was unique, there is a bit of in-canon One Piece evidence to support this theory.
While a rare occurrence, there have been multiple cases of fetuses absorbing one another while in the womb before they are born. This is commonly referred to as a "Vanishing Twin." This process essentially creates a person with multiple sets of DNAs, which can result in abnormalities that remain within the body after birth -- even potentially giving that fetus multiple sets of organs.
It is possible that Blackbeard's body actually has multiple "beings" inside, which is why having multiple Devil Fruits doesn't kill him. If a person can only have a single Devil Fruit, and he is made up of multiple people, then it is entirely possible Blackbeard's body could utilize as many Devil Fruits as there are people within him.
Tremor-Tremor Myth
Blackbeard cuts off Devil Fruit user powers.
This One Piece theory is unique because it almost entirely steps away from the idea that Blackbeard's body is responsible for his ability to use multiple Devil Fruit powers. Instead, this One Piece fan theory focuses on the Tremor-Tremor Fruit, and it may explain why Blackbeard was willing to kill Thatch to seize this fruit. This particular fan theory also supports the idea that Blackbeard's Devil Fruit is as special as he claims it to be.
Since his fight with Ace, Captain Blackbeard has stated that the Dark-Dark Fruit is unique among Devil Fruits, even among the Logia types. While it does not offer him the same elemental intangibility that Logias are famous for -- and actually makes him much more susceptible to damage -- Blackbeard clearly stated that his Devil Fruit's other abilities easily compensate for that. For example, Blackbeard's mastery over darkness has granted him a degree of control over gravity as well, allowing him to draw objects and people closer to him against their will. He can even absorb things into his darkness, later expelling them with great force.
The Dark-Dark Fruit's greatest power, meanwhile, was its unique ability to shut off another Devil Fruit user's powers with physical touch, which Blackbeard first showed during his battle against Portgas D. Ace. Many fans believe it's this power, or an extension of it, that allowed Blackbeard to take Whitebeard's Devil Fruit abilities. If so, this theory could very easily explain just why Blackbeard was so keen on having this particular Devil Fruit, as he even admitted he waited for decades aboard Whitebeard's ship hoping to find it.
The Vegapunk Effect Theory
Blackbeard's body was enhanced by Dr. Vegapunk.
Perhaps the most farfetched theory on why can Blackbeard have 2 Devil Fruits is the Vegapunk Effect, which proposes the idea that Vegapunk enhanced Teach's body to be able to use multiple Devil Fruits. This theory is considered among the less likely, as it would require a lot of different details to prove it true. Most notably, it would have required the infamous Dr. Vegapunk, who has worked for the World Government for years, to have met Blackbeard some time before the events of One Piece.
Given what little is known about Dr. Vegapunk, it's difficult to fully prove or disprove the Vegapunk Effect theory. After the Straw Hats were separated on Sabaody, Franky lands on Karakuri Island, a winter island on the Grand Line where Vegapunk grew up. It has been confirmed that Blackbeard was born on a Grand Line island as well, though the exact location hasn't yet been revealed. If any experiments occurred, they most likely would've been on Karakuri Island, but without knowing where Blackbeard is from, it is impossible to say for sure. While it is possible these experiments could have happened after Teach joined the Whitebeard Pirates, it's highly unlikely given that the World Government would have known of the threat he posed.
There are many potential explanations for how Blackbeard can wield two Devil Fruits in One Piece. In fact, if he can wield two, then there's no reason to believe he can't also wield three, especially given Blackbeard's -- and the series as a whole's -- fixation on the number three. Blackbeard even uses three pistols and has three missing teeth.
The Cannibalism Theory
How does Blackbeard have 2 Devil Fruit powers? The most gruesome fan theory posits that Captain Blackbeard resorted to cannibalism to obtain the Tremor-Tremor Fruit's powers from Whitebeard's corpse without having to wait for the Tremor-Tremor Fruit to reappear somewhere so Blackbeard can eat it. Fans can compare this to a young Charlotte Linlin cannibalizing Mother Carmel by mistake on Elbaf, which is evidently how Big Mom got her Soul-Soul Fruit powers from Carmel. It seems a Devil Fruit's supernatural essence is absorbed right into the user's flesh and blood, and consuming enough flesh and/or blood will transfer those powers. In Blackbeard's case, he may have drank some of Whitebeard's blood ar Marineford, or even tore out Whitebeard's heart through an existing wound and consumed it.
If the cannibalism theory alone isn't enough to convince One Piece fans about why can Blackbeard have 2 Devil Fruits, then this obscure theory can be easily integrated into other theories. If Blackbeard has Cerberus Devil Fruit powers, for example, he can get other Devil Fruits either by eating the fruits directly or cannibalizing another Devil Fruit user to get their powers. If so, Blackbeard chose cannibalism for Whitebeard's powers simply because Blackbeard didn't want to wait for the Tremor-Tremor Fruit to regrow somewhere -- he wanted the power right away. Finally, as for why no one else in One Piece aside from Big Mom and Blackbeard has used cannibalism to seize Devil Fruit powers, it's possible that this transmission method simply isn't widely known, with Big Mom stumbling upon it by accident and Blackbeard, for the plot's sake, being the only pirate who actually learned the truth of Devil Fruit cannibalism, so he can put it to use.