Phasing is the natural phenomenon that allows multiple worlds to intersect and interact with each other in a limited fashion.
It is not clear if phasing is a cause of the Excessions, or is a result of them, but it is becoming clear from current research that phasing occurred very rarely, if ever, before the Primary Excession.
Scholars have theorized that there are two primary narrative universes (or verses) that appear to be phasing into one another. The first (and the one most consistent with Pre-Excession history and causality) is commonly called the Excessioniverse, while the other is dominated by a centuries-long series of wars between humans and robots. This is called the Robowariverse. Certain events, items and individuals appear to exist in both verses. The popular TV series Dinglebots! is firmly grounded in the Excessioniverse, but appears to depict events in the Robowariverse with some degree of accuracy. There are also two sets of brothers that appear to have links to both verses -- the Droolys and the Bantus. It is possible that they are, in fact, the same set of brothers with different names in each verse.
It is also believed by prominent quantum narratologists that it is possible to separate the two universes by creating a powerful device known as a Quantum Narrative Flux Generator. This device would create a charged field of narrative particles (known as "plotons") that would force everything into its own verse, thereby separating the two realities for good. As most of the world's quantum narratologists have joined the Polished Mirror Institute of the past several years, it is rumored that the secretive Institute is working on such a device currently.