>>281961126
BB's body is what gave him the ability to eat two fruits. It's plainly alluded to in Marineford when Marco comments about it.

Darkness fruit allows him to capture its powers to prevent the random respawning devil fruits do. That's why he covered himself with a tarp when extracting the Quake fruit from Newgate's corpse. If anything he stores fruit in his void so the DFs extracts from dead people automatically go to his stash.