I, too, believe that this cerberus theory is valid. Even if you look at his pirate flag, there is a formation of 3 skulls. strawhat pirates have one skull with strawhat, shanks has one skull with 3 scars, whitebeard one skull with beard, big mom skull with hair and lips, buggy skull with nose,
so why 3 skulls on blackbeards flag.
further I don't think his devil fruit gave BB the ability to take the gura gura no mi, because CC doesn't have the yami yami no mi and can create new devil fruits, and doflamingo got ace's fruit, but I don't think they digged the corpse out for retrieving the fruit -.-
so I really do think and hope, that BB is actually not 2, but 3 people.