This is a little wrong, my thoughts is that it's the one God, split into the Outer Twins when it died, then they made the 6 Arbiters, each of which split in two when they died to become the 12 Apostles. When a being with a complete power dies, like God or the Arbiters, they split into their base parts, allowing them to still live on. Those incomplete halves cannot split when they die, they completely die. That would explain how some Apostles are supposed to be dead for real.
The scene I was referring to before, where we are listening to Chaos' memory, goes as such, "I see them. The enemy. Our kin. Twisted. We are the last. The only three to have prepared. The others were taken as newborns and now... Six. They should be lesser... but are they? Twins... their nature... Our preparations came to naught. We acted to late. All must begin again. Chaos. Burns it all down. Rebirth. The time will come. Our ashes will pave the way. We three... And one other." That, to me, implies some kind of outside enemy, along with the likelihood that three Arbiters and one Outer Twin were left after some attack by said enemy. This also explains my reasoning about God, the twins by themselves are kind of lesser, but if they come together, they can bring forth the original power of their Arbiter. That could easily be extrapolated to the Outer Twins, that if you could bring their powers back together, you could recreate God's might in full.
It could be that Ella is the key, however, there is a problem with that for me. During the Dead End you are talking about, we come face to face with Ella in that state, and we have no idea who Ella is. She's even sad about it before Nico teleports us away to our loss against Deus. It may be that Ella is the one who can prepare the real ritual that allows things to get real nutty, after all, Doll Ella says something like, "The real party can't start without me," in reference to her dead original self.