We have seen that souls and devil fruits can be imbued into inanimate objects with similar effect.
Maybe devil fruits have a direct connection to the soul. Maybe Vegapunk didn’t find a way to get objects to eat devil fruit, but found a way to give them artificial souls that could contain a devil fruit.
One soul can only contain one devil fruit, so with this theory Blackbeard must have two souls. Luffy and Zoro could have been sensing this with haki when they called him more than one person.
A Strzyga is a demon somewhat similar to vampire in Slavic (and especially Polish) folklore. People who were born with two hearts and two souls and two sets of teeth (the second one barely visible) were believed to be Strzygas. Marco did say the structure of Blackbeard’s body was strange.
Could Blackbeard be a Strzyga? His three skull Jolly Roger could represent his two souls and the Strzyga beast form.
There are some connections between Strzyga and real life Blackbeard too. A Strzyga’s second soul stays in the body after death and it comes back to life. Even decapitation won’t stop this. It was said you should decapitate the Strzyga and bury the head separate from the body. There is a legend that real life Blackbeard’s decapitated body swam around a boat three times before sinking.
*Theory by Devil_Fruit_Salad