Philip Jose Farmer's "Wold Newton" concept has been a major influence on crossover-fiction. The idea first appeared in his two "imaginary biographies," TARZAN ALIVE (1972) and DOC SAVAGE: HIS APOCALYPTIC LIFE (1973), in which Farmer wove a complicated web of interrelationships between the titular heroes and a wide number of other fictional and real-life personages. All of these figures were loosely related through ancestors who had been exposed to a radioactive meteor that fell to earth in Wold Newton, Yorkshire, in 1795. The meteor's radioactivity mutated various humans, so that many of their descendants inherited high levels of physical ability and/or mental acuity.
Influential though these books were, neither of them is actually a crossover between two or more centric characters: they are pseudo-histories devoted to making what I called "allusion crossovers"-- and therefore not admissible here.
But then there's this novel, published the same year as the DOC SAVAGE history:
THE OTHER LOG OF PHILEAS FOGG does not invoke the meteor-theory, though Farmer may have had it on his mind when he conceived the novel. Farmer's conceit here is that the assorted inconsistencies he Farmer finds in Jules Verne's only novel to feature Fogg, 1873's AROUND THE WORLD IN 80 DAYS, are explained by "another log" that reveals that Fogg was actually an alien-human hybrid, descended from an intermarriage between an Earth human and an alien called an "Eridanean." Other Eridaneans form a spy-network on Earth, where they can easily pass as human beings as they attempt to thwart the plans of the evil Cappelleans, who have more ruthless plans with regard to humanity. Just like the Eridaneans, all Cappellean agents are human-alien hybrids, the most noteworthy being Jules Verne's most famous character, Captain Nemo of 20,000 LEAGUES UNDER THE SEA and THE MYSTERIOUS ISLAND. Two other Verne characters from AROUND THE WORLD, Passepartout and Aouda, are said by Farmer to be part of the Eridanean network.
OTHER LOG is a good, readable adventure, even though it can be a little tedious whenever Farmer offers his nitpicking critique of the original Verne novels. But though I can't say that I liked Farmer's re-interpretation of Nemo-- who becomes a simple "pirate" rather than a Byronic hero, and who is even to be coterminous with Sherlock Holmes' main villain Moriarty-- I have to say that for the most part Farmer does understand the appeal of both Verne characters, and so often a satisfactory interaction of their unique personalities.