Crossover writer's manual - a suggested guideline.

[Heaven's Deamon]

Indeed, another example of what you're talking about would be Bleach and YuYu Hakusho. Spirit World and SS serve the exact same purpose of being the afterlife, and therefore it would be extremely difficult to have them exist in the same world.

[Himitsu The Hunter]

Indeed, another example of what you're talking about would be Bleach and YuYu Hakusho. Spirit World and SS serve the exact same purpose of being the afterlife, and therefore it would be extremely difficult to have them exist in the same world.

You are correct that it such a thing would be difficult, though in that particular case i can see how it would be done... In YuYu you never actually see Enma himself, Or much of spirit World itself, so you could go about saying that spirit world which governs Humans is then split into The Palace (where the king resides), Soul Society (where they place positively spirited humans in their afterlife preparations/ gain and train Shinegami), Hecomundo (where the negatively based humans... the hollows and such are put), and Hell (where they Punish negatively based humans). Then Have the entire spirit world demon world conflict as a completely different set of problems, thus neatly divvying up the misery of the combined world setting without actually getting in the way of the actual stories they've already told.
On a side note though... Inuyasha/Yuyu Hakusho fanfics have annoyed me for a long time... for 1 main reason. They always seek to break up the established cannon couples and then pursue to couple together KAGOME/KURAMA... it was amusing the first 2 times... but now when i see that I roll over and go find a new fic, cause the theme seems to be the ONLY one explored and it drives me nuts that i cant get to read a good crossover between the 2 without it devolving into a Kagome/Kurama mush fest....
It essentially turns the established characters into marysue puppets for that particular writer's stings... I guess iI'm just a bit upset that i can only find about 4 other Crossover fics in that category that didn't do this... and none of them were finished........
-Himitsu The Hunter