As the tower began to collapse from Romeo's pre-set charges, they reached the roof. Only one escape pod remained. In a typical story, they would fight over who stays. In this one, they realized the ultimate truth: The Origin Core they stole in Chapter 3 wasn't a battery. It was a consciousness-splicer.
During their time in the vault, the device had secretly begun merging their neural patterns. They weren't just "working together"; they were literally starting to share thoughts. Romeo felt Juliet's childhood trauma; Juliet felt Romeo's hidden loneliness. The hatred was a byproduct of their minds trying to reject the merger.
The twist? They weren't two people anymore. They were a single consciousness in two bodies. To kill the other would be to lobotomize oneself.