Doomsday Clock SPOILERS
It just occurred to me that the Metaverse reveal at the end of DDC might finally explain the whole 'two' Supermen situation from Convergence/Superman Reborn.
Here goes-
1. Pre-Flashpoint Superman and Lois exist on Earth 0.
2. Doc Manhattan's tampering alters Earth 0. Pre-Flashpoint Superman and Lois become New 52 Superman and Lois.
3. As a result of Manhattan's tampering, a new parallel earth is created to 'preserve' the Pre-Flashpoint continuity. Pre-FP Superman and Lois continue their lives on this earth.
4. During Convergence, Brainiac grabs the Metropolis from this parallel earth, including Pre-FP Superman and Lois. During their time imprisoned in the bottle city, Jon is conceived and born. Post-Convergence, Pre-FP Superman and Lois end up on Earth 0 and raise Jon there.
5. New 52 Superman and Lois die, and the Pre-FP versions take their place.
6. The essence of the New 52 Superman and Lois 'merge' with the Pre-FP Superman and Lois. This alters the history of Earth 0, restoring its earlier Pre-FP configuration to a large extent.
So basically, the 'split' of Superman into two was caused by Manhattan simply because Manhattan tampering with Earth 0's history caused the creation of a parallel earth that preserved the earlier version of Superman. Pre-FP Superman, despite being from a parallel universe, was essentially a previous timeline version of New 52 Superman - thus he was able to be 're-incorporated' into Earth 0 history, becoming Reborn Superman.