Among One Piece's trademark features are the Devil Fruits, items that grant their users permanent superhuman powers of the most varied types in exchange for their ability to swim. Depending on the type of ability they provide, Devil Fruits are grouped into three main categories: Paramecia, Zoan, and Logia.
Throughout their entire lives, One Piece characters can only eat a single Devil Fruit. Any attempt to eat a second one and gain another ability will immediately cause the consumer's death. Only Marshall D. Teach "Blackbeard" managed to wield two powers at the same time through unknown means.
If it were possible to simultaneously use two Devil Fruit powers on a normal basis, some especially advantageous combinations could be performed. This article lists five hypothetical Devil Fruit combos in One Piece that would be absolutely deadly.
Disclaimer: This article contains major spoilers from the One Piece manga up to chapter 1083.
Flower-Flower Fruit + Sand-Sand Fruit and other Devil Fruit combinations that would be deadly in One Piece
1) Flower-Flower Fruit + Sand-Sand Fruit
The Flower-Flower Fruit (current user: Nico Robin) allows the user to sprout duplicates of their body from any surface within range. Meanwhile, the Sand-Sand Fruit (current user: Crocodile) enables the user to create and control sand.
Among the various abilities granted by the Sand-Sand Fruit, the user can dehydrate their foes until they get mummified. Using the Flower-Flower Fruit, this move could be performed very effectively, using the body-sprouting power to take the enemy off-guard.
The Flower-Flower Fruit's replicated body parts are an extension of the user's body, which means that if the duplicates are damaged the user's real body would be injured. However, this weakness would be fixed with the Logia-class powers of the Sand-Sand Fruit, which turn the user's body into intangible sand.
2) Clear-Clear Fruit + Hobby-Hobby Fruit
The Clear-Clear Fruit (current user: Shiryu) allows the user to become an "Invisible Man." Meanwhile, the Hobby-Hobby Fruit (current user: Sugar) enables the user to gain eternal youth and gives them the ability to transform living people into toys, erasing any memory of their existence from others.
To transform other people into toys, the user of the Hobby-Hobby Fruit needs to physically touch them. Depending on the opponent's power and the user's own physical prowess and fighting capabilities, this may prove to be difficult and risky.
However, if the user of the Hobby-Hobby Fruit were to exploit the abilities of the Clear-Clear Fruit, they could turn themselves invisible to approach their target without being detected.
3) Dark-Dark Fruit + Bird-Bird Fruit Model Phoenix
The Dark-Dark Fruit (current user: Blackbeard) allows the user to create and control darkness, conjuring a black void that devours and crushes everything. The Bird-Bird Fruit Model Phoenix (current user: Marco), on the other hand, enables the user to self-regenerate any kind of damage as long as they have enough stamina left.
The Dark-Dark Fruit also allows the user to completely nullify the abilities of other Devil Fruit users by simply touching them. Still, despite being a Logia-class power, it does not give its user access to an intangible elemental state, thus leaving them exposed to being struck and possibly injured.
To make things worse, it seems that the user of the Dark-Dark Fruit suffers double the amount of pain for any damage they receive. Using the regenerative powers of the phoenix, however, they could heal immediately.
4) Human-Human Fruit Model Nika + Chop-Chop Fruit
The Human-Human Fruit Model Nika (current user: Luffy) allows the user to become an incarnation of Nika, the legendary Sun God, turning themselves into a "Warrior of Liberation" who can fight freely following his imagination.
This Devil Fruit gives the user's physique the properties of rubber. It provides them with immunity to blunt force, including hand-to-hand strikes, blunt weapons, bullets, and even cannonballs, which will just bounce off from their rubber-like skin. However, they would still be vulnerable to slashing attacks.
The Chop-Chop Fruit (current user: Buggy) enables the user to split their body into pieces and reassemble them at will. They will become immune to slashing attacks but remain susceptible to damage from blunt attacks.
Combining the powers of these two Devil Fruits, their respective weaknesses would be mutually fixed, obtaining complete immunity to any kind of physical attack.
5) Glint-Glint Fruit + Slow-Slow Fruit
The Glint-Glint Fruit (current user: Kizaru) allows the user to create and control light, as well as transform their body into it. Meanwhile, the Slow-Slow Fruit (current user: Foxy) enables the user to turn themselves into a "Slowpoke Human" who can emit Noroma Photons, special particles that slow down any object they hit for 30 seconds.
Be it people, objects, or even the space itself, anything touched by these particles will be slowed down. Still, to avoid any unfavorable effect, it is sufficient to dodge the Noroma Photons.
Using the Glint-Glint Fruit to move and attack and the speed of light, the user would render the techniques of the Slow-Slow Fruit almost impossible to dodge due to the immense quickness they would be performed at.
Final Thoughts
Normally, it's impossible for One Piece characters to eat two Devil Fruit, wielding two different abilities simultaneously. As such, despite being theoretically very effective, all the aforementioned combinations will never be featured in the series.
However, One Piece fans have always been very intrigued by Devil Fruits and will never stop speculating about possible combos between different abilities.
There is one combo between Devil Fruits that is actually canon, and it's absolutely tremendous. Combining the Dark-Dark Fruit and the Tremor-Tremor Fruit, Marshall D. Teach can pull his foes into his grasp and then strike them with a devastating quake-enhanced punch.
In the series, the strongest power and the deciding factor of most battles is Haki, which surpasses even Devil Fruits. The legendary Gol D. Roger didn't need a Devil Fruit to become the Pirate King. He conquered the Grand Line with his Haki.
No fighter ever became particularly powerful without being a skilled Haki user, while many characters have achieved notable might even without relying on Devil Fruit powers. According to Kaido, an individual with strong enough Haki can potentially overcome any hurdle and win the fight.
Still, Devil Fruits remain a major asset for One Piece fighters, as they enable them to use a plethora of varied abilities. This is especially true for users who achieved a stage called Devil Fruit Awakening, which allows them to handle the item's full potential, gaining powers on a whole different level.