It has been stated that one cannot eat 2 Devil fruits (with the exception of Blackbeard's uncanny unexplained body), because the body can't handle it and a person would die upon eating the second DF. But what if that person can't die? For example, take Kaido who as we know is immortal to some extent, I wouldn't put it past Oda to give him 2, maybe even 3 different Zoan types! That'll be a real threat, especially if one of them is a mystical type.
Another example would be with healing/revival. Say a person ate the second fruit and as they're about to die Princess Mansherry heals their ailment so they don't die? Or what if the person eats both, dies, then is brought back to life with some fruit power like Gecko Moria's? For all we know he could put someone's shadow upon the corpse of the 2 fruit person (granted I understand that once a person dies the fruit they ate would somehow leave their body and reincarnate elsewhere, but this is all theoretical).
Last example is Brook. As we know he ate the revive fruit, which enabled his soul to remain in his body after his first death, but what if he ate a second fruit before he died? He would revive because of the initial fruit and hopefully his body would retain the second fruit, therefore giving him a second DF! Now after death it is unlikely that he could eat a second one since,
1. It might end his second life and,
2. He can't eat a fruit since he has no stomach (Yohohohoho)
But think about it, it is possible. And remember how he died? Him and the rest of his crew all eventually died of starvation on their ship, but who's to say that perhaps on that ship, after all the reasons were gone, there only remained one other edible item... another devil fruit! Whether by trying his luck or as a suicide attempt, Brook could've eaten a second fruit, harboring and hiding it from others until he mastered it!
But again, it's just a theory.
Sorry for the disertation, but thank you for reading. If you agree or come up with any thoughts or flaws concerning this please lmk.