Now, I know the Gorosei said there are no records of anyone ever eating two Devil Fruits before until Blackbeard but what if till this day no one has ate two of them?
What if Blackbeard absorbed the “power” after Whitebeard’s death and it had nothing to do with eating it?
What if Blackbeard was NOT the first person to gain the power of 2 devil fruits, and he was just the first person to show this ability? We have recently saw proof of the Clan of D. using two devil fruit abilities with Luffy and Law too. All three of them absorbed a part of the powers shortly after someone close to them died.
After the death of Whitebeard, Blackbeard absorbed his powers consciously/by force (possibly using his Devil Fruit powers). But Luffy and Law did it subconsciously. Or maybe Rosinante and Ace’s will was passed on in the form of the Devil Fruit powers but Blackbeard left the will behind and absorbed only the power.
In short, I think this is an ability that the D. Clan have. It is a way to pass on their will and/or for another to carry it forward.
The first person to absorb someone’s Devil Fruit power was Law after Rosinante died. He showed this ability on Onigashima when he “silenced” Big Mom. He used Rosinante’s powers/technique.
Luffy was holding Ace when he died and later gained the power of fire. He showed this ability after the time skip.
Although these powers were passed on or absorbed it is still possible that the fruits were still regrown elsewhere just like Ace’s devil fruit. I think they only absorbed a portion of the powers or techniques.
I also speculate that if someone without any devil fruit powers from the D. Clan absorbs the power in this way that they would not suffer the curse of the devil fruit itself. They would still be able to swim.
I am not saying this is fact but in my opinion it is a possibility.
*Theory by three-day-trip