WARNING: The following contains spoilers for Chainsaw Man #91 by Tatsuki Fujimoto, Amanda Haley and Sabrina Heep, available in English from Viz Media.
Chapter #91 of Chainsaw Man features the touching departure of Power, as she Denji gives her life for his, forging a contract with the young Devil Hybrid. The terms of the contract: Denji has to find Devil Power in Hell and return her to the friend he once knew.
Touching though this scene may be, it begs the question: does Denji have two sets of Devil powers now? Is that even possible? Chainsaw Man throws plenty of curveballs, but there's no real precedent for someone being a Hybrid of two Devils. On top of that, since Power ate Pochita to get stronger in chapter #90, does that mean Pochita is no longer inside Denji? These are complicated questions with surprisingly simple answers.
Turns out, the answer to most of these questions lies squarely on Denji's chest -- literally. Shortly after Power leaves for good, Denji emerges from the dumpster she hid him in, bare-chested. Right where his heart should be, as it always is, is Pochita’s pull-cord, still intact. Since it's Power's blood that brought Denji back to life, this should mean that, in addition to having her blood and whatever abilities that comes with, Denji also retained his Chainsaw Hybrid powers.
But this revelation is at odds with everything seen so far regarding Devil Hybrids. Every Hybrid thus far had the powers of just one Devil. Even those friendly with multiple Devils, like the Devil Hunter Quanxi, are only fused with one. But that doesn't mean it's not possible. Humans and even other Devils, like Makima, are consistently seen contracted to and controlling multiple Devils, meaning that the creatures have no qualms about sharing. So, given the inherent rarity of Devil Hybrids in the first place, it's possible that they can have two power sets -- it's just even rarer.
But then there's the actual mechanics behind Denji getting two sets of Devil powers. At first, it seems as though something should have been lost in the exchange. After, Power ate Pochita and then died, so -- in theory -- Pochita should be gone. However, the Chainsaw Devil was likely able to live on in the same way that Power was originally able to return to life. The only reason Pochita was able to reach out to Power is that her blood already flowed through Denji's veins. So, the first time Makima killed her, there was a shred of herself still living that she could come back from (though she needed a boost via Pochita since Denji was bleeding out).
Likewise, since part of Pochita was still serving as Denji's heart, even though Power ate him, he was still able to maintain a presence in Denji's body. Another explanation could be that Pochita did leave Denji's body for Power's, returning via the blood she gave to Denji. In other words, when Power fused with Denji, Pochita came along, too. However, one could argue that Pochita leaving Denji's body at any point would've instantly killed him.
In any case, Power's contract and Pochita's pull-chord are proof enough that Denji is currently a Hybrid of a human and two different Devils. This unexpected turn of events has discreetly revealed more about how Devil Hybrids work, including pointing to the fact that, as long as a Devil exists somewhere in some form, the contract is still valid and the powers are kept. This also means that there are technically two Blood Devils out there -- the reborn Power and Denji the new Blood Devil Hybrid.
Power's, uh, power infusion came at just the right time, too. Thanks to Makima's scheming, Denji's Chainsaw powers are severely weakened. Devils are empowered by fear but, since the people see Chainsaw Man as a hero, no one's afraid anymore, dulling his fangs. But people are still plenty frightened of blood, meaning Denji still has enough juice in him to make a change... of some kind.