On Shared Universes
On "shared universes" it is always a question of more or less —just as in "real life" we share our universe to a certain extent, always partially so, but perhaps never in a complete way. In the case of fictional worlds, an explicit reference to characters or events in another novel is taken as a sign that the author wants to emphasize the continuity between both novel worlds, and this may be either central or anecdotal (perhaps just a mark of the author's personal affection for his own tiny "comédie humaine"). But in the last analysis, all human universes, fictional or not, are partially shared by the fact that we live in a common and interconnected semiosphere. If there were any universe which was completely autonomous or non-shared, not resting on a common ground with our universe, then that's an issue similar to the multiverses in cosmology. They are a mathematical or logical problem without any demonstrable physical connection to our own universe. That is, if we make abstraction of the fact that these problems have been thought out IN OUR UNIVERSE, and in that sense they are also subordinate hypothetical worlds resting on the common world of shared experience.
Or at least that's the way it looks if we see it from here.
A paper (in Spanish) on the multiverses of Stephen Hawking and Olaf Stapledon in The Great Design and Star Maker: http://papers.ssrn.com/abstract=2535379
From a thread in the Narrative-L. David Richter adds:
Sometimes a writer creates a world which is largely coherent but with occasional inconsistencies, of which the most canonical is Faulkner's Yoknapatawpha novels. Kirk and Klotz in Faulkner's People (1965) index all the characters, with special treatment for ones that are handled inconsistently. One example of inconsistency involves Quentin Compson, who in The Sound and the Fury (1929) commits suicide while at Harvard at the age of 19--the Anderson Bridge over the Charles River actually has a plaque commemorating this fictional event; but Quentin also narrates the short story "That Evening Sun," from These Thirteen (1931), and he is 24 years old as narrator (9 years old at the time of the story which is timestamped "fifteen years ago").
Close readers can overread these inconsistencies through elaborate midrash that creates implausible storyworlds. The Baker Street Irregulars, who read the Sherlock Holmes stories ("the canon") as though they represent a coherent and consistent world have presented theories that Watson (who marries Mary Morstan in The Sign of Four) was married three or four times because the dates of action of particular stories (as inferred from details in the stories) suggest that Watson was married, then unmarried, then married again, then unmarried.... Or that his middle name is Hamish (Scottish for James) because his name is John H. Watson and his wife, at one point, calls him "James." I should mention that such research findings generally have their tongue firmly in their collective cheek.
Similar close reading in Biblical studies takes place without irony. For example, Isaac has been determined to be 37 years old at the time of the aborted sacrifice in Genesis 22, though his age is never mentioned in that chapter and the dialogue with Abraham would suggest that he is a boy of ten or so. The inflexible logic of 37 proceeds from Sarah's age at Isaac's birth (90), and her age at her death (127) at the beginning of Genesis 23. Since Sarah is not mentioned at all in Genesis 22, it is inferred that the cause of her death was finding out--in some midrashim from an evil angel--that Abraham has gone off to sacrifice Isaac--and if that is so Isaac must be 37. Similar midrash is created in New Testament studies to reconcile inconsistencies in the Gospels, e.g. who carries Jesus's cross to Golgotha: the Gospel of John says that Jesus carried it; the other three (synoptic) Gospels say it was carried by Simon of Cyrene. To remove the inconsistency, the midrashic process creates an emplotted event (Jesus falls while carrying the cross, the Romans find a man in the crowd, Simon, and draft him to help) that is so vivid that it is often pictured in religious paintings (like Breughel's Way to Calvary) and has been filmed in practically every Jesus movie I know of.
Some aspects of this fictional-world-managing are dealt with by Marie-Laure Ryan under the heading of "transfictionality", e.g. in "Transfictionality Across Media." In Theorizing Narrativity. Ed. John Pier and José Ángel García Landa. (Narratologia, 12). Berlin and New York: Walter de Gruyter, 2008. 385-417. On the other hand, the mental or discursive operations involved in "fitting" a narrative within a larger narrative, historicizing it, or building an overal chronology out of disparate narratives, may be seen as an aspect of what I call "narrative anchoring." See e.g. here, "Harry Thompson, 'This Thing of Darkness': Narrative Anchoring" https://www.academia.edu/336349/Harry_Thompson_This_Thing_of_Darkness_Narrative_Anchoring