Canonically, John Constantine is a survivor twin because he strangled his twin in the womb. It's unnecessarily try-hard edgy. We changed his birth to revolving around Vanishing Twin Syndrome and it results in John being a Chimera twin. Because he's an identical twin, this means his chimerism is harder to detect. So he uses that to his advantage to make deals with a dead soul mixed with his own.
I think this adjustment would make more sense for his character! A common theme throughout Hellblazer is the idea of "Surviving But At What Cost" and that someone has to be sacrificed so John can survive. There's a lot of guilt and he's haunted by those he had to use to survive, so survivor's guilt over twin loss feels so at home for him. The difference here is the loss he experienced from before birth was borne out of love instead of hate. I just find that more bittersweet than a baby hating for no reason.
