Few One Piece characters are as vile as Marshall D. Teach, commonly referred to as 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 Yami Yami no Mi. If that wasn't bad enough, Blackbeard captured Portgas D. Ace and handed him to the World Government in order to gain the status of a Warlord. This is all so he can cause a war that would allow him to acquire the crew he wanted, and to kill Whitebeard and steal his Devil Fruit powers.
Curiously, Blackbeard is the first human in One Piece history capable of wielding two Devil Fruit powers at once, supposedly an impossible feat. CP-9 agent Jabra himself reveals this is the case though, 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. But just how was he able to do so? Here's a deep dive into the most popular theories.
Cerberus Theory - Blackbeard Had Already Eaten 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 Yami Yami no Mi. Specifically, a Mythical Zoan Fruit possibly called the Inu Inu no Mi, Model Cerberus. Theorists believe that due to the nature of the actual mythical Cerberus having three heads with independent thought, this allows Teach 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.
First is Blackbeard's Jolly Roger, meant to represent a ship's captain. His specific flag features the standard skull and crossbones, but also 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 directly in One Piece's world gives this theory even more credibility.
Chimera Theory - There Are Multiple Entities Inside Blackbeard
Much like the Cerberus Theory, the Chimera Theory operates on the idea that there are multiple entities within Blackbeard. 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 quite 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 DNAs and sets of genetics, which can result in abnormalities remaining within the body after birth. This can even potentially give that same fetus multiple sets of organs.
It is quite possible that Blackbeard's body actually has multiple "beings" inside of him, which is why having multiple Devil Fruits doesn't kill him. If a person can only have a single Devil Fruit and Blackbeard was actually made up of multiple people, then it is entirely possible his body could utilize as many Devil Fruits as there are people within him.
Yami Yami no Myth - Shutting Off a Devil Fruit User's Powers
This One Piece theory is unique in that it almost entirely steps away from the idea that Blackbeard's body is responsible for his ability to use multiple Devil Fruit powers. Rather, it comes from the Yami Yami no Mi, which is why he was willing to kill Thatch in order to get it. The Yami Yami no Myth also supports the idea that Blackbeard's Devil Fruit is as special as he claims it to be.
Since his fight with Ace, Blackbeard has stated that the Yami Yami no Mi is unique among Devil Fruits, especially Logias. While it does not offer him the same elemental intangibility that Logias are famous for -- and actually makes him much more susceptible to damage than he would be without it -- Blackbeard clearly stated that its other abilities more than make up for it. For one, his 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 is even able of sucking things into his darkness, later expelling them with great force.
Its greatest power, however, was its unique ability to shut off a Devil Fruit user's powers, which Blackbeard first showed against Ace. Many fans believe it's this power, or an extension of it, that allowed Teach to take Whitebeard's Devil Fruit abilities. If this were the case, it could very easily explain just why he 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.
Vegapunk Effect - Blackbeard's Body Was Enhanced
Perhaps the most farfetched theory 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 hard to deny or accept this theory's credibility. 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, it 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 the character's -- as well as the series as a whole's -- obsession with number three. Blackbeard even uses three pistols and has three missing teeth. Whether the true reason is good or bad, one thing's for sure -- One Piece loves to take its time getting there.