The Count is really Daishuryou

So, JTTM is split between many Multiverses, though there are only two that are massive: the original JTTM and the re-imagined JTTM for future RPGs. Think of it like how in the Delta Episode of Pokemon ORAS, you had the two Pokemon Worlds, one with Mega Evolutions and one without.

So here's where the whole "fueling the other" comes into play. Somehow, some way, the Daishuryou finds himself defeated and sealed away into another world. For good measure, it's another world in a different plane, where the Monitors of the Multiverse are completely different, to the point of being just Gods. This is the re-imagined JTTM.

Daishuryou winds up being just an essence now. This essence then got leather-bound into a book called the Dark Prognosticus and handed to an everyday schmuck who became the new Great Leader, though with the essence of Daishuryou merged with his consciousness instead of full on possession. This becomes Count Logan, a mysterious villain who is said to either be the Messenger of Yvetal or the Ultimate Despair. Depends on who you ask, really.

Daishuryou pretty much becomes the book. Anyone who comes into possession of it, if they have an empty heart (i.e. lack one or some equivalent of, or their heart is so dark that it can be considered empty), Daishuryou is more than welcome to merge with the holder to become a new Count.