Reality has split-and it's Definitely Not Kez's Fault.
There are Fixed Points in the universe: places where events Must Always Happen, no matter what else changes in the timeline. But Someone isn't happy with the way things turned out.
Someone knows that if those Fixed Points are disturbed, another version of reality can be created.
And Someone is desperate enough to risk destroying the universe just for a chance to make their own vision into reality: a vision that will separate Kez and Marx from each other and all that they hold dear.
Marx without Kez is a small, angry man with a burning desire for revenge, but Kez without Marx is a feral bundle of rage and fear that could well destroy any version of reality.
Someone really should have thought of that before they began splitting reality...