My point isn't to beat up on an easy target. This is an extreme example of how not to mix storylines. Batman and Superman team up to save Earth from the Aliens and Predators. Keeping the Canon StraightSuperman and Batman live in Gotham City and Metropolis, which look pretty much like present-day American cities. The Aliens and Predators inhabit a universe which includes an Earth several hundred years advanced from ours. In fantasy, this isn't an insurmountable obstacle, but you can't just let it go unexplained how Batman and Superman came to inhabit the Alien/Predator universe. I read comics voraciously as a pre-teen, and believe me: this is a question that would definitely occur to a twelve-year-old comic fan.One Story Underminines AnotherThe Alien stories are packed with drama because every contact with them is a life-and-death situation. The Aliens are virtually indestructable maneaters; if they ever reached Earth, they would represent an existential threat. Batman, on the other hand, is mortal with no superpowers, who mostly fights streetcrime. When you have Batman kicking the asses of four Aliens (as he does on page 24-25), the reader has to step back and say "Wait a minute! maybe the Aliens aren't very tough at all. Why have I been reading this?" Add Superman to the equation, and the Aliens are little more than a nuisance. That's no good for Aliens fans. Publishing comic books is a business, and writing crossover stories is a strategy to expose readers to franchises they may not have visited on their own. "If you liked Batman, you might like Aliens." The strategy backfires if it turns fans off of a series they had grown loyal to.Doing Crossover RightMixing established characters from different series needs to be done in a way where the storylines complement each other. The Marvel and DC comics frequently do this well. The 1976 Spiderman/Superman crossover is very complementary; Spiderman has a darker, more cynical side that is just "cooler" than Superman, but Superman has a nobler, more optimistic outlook that can be uplifting when it isn't overdone. Another approach to writing crossovers is for humor. ODDBALL COMICS likes doing juxtapositions that are so ridiculous they are noncanonical, like Archie Meets the Punisher. That's fine, but Batman/Superman/Aliens/Predators isn't quite ridiculous enough to be parody. (same goes for Xmen meet Captain Kirk ,by they way) Parody gives you a lot of license, but even then, you have to have characters that appeal to the same readers, which is why you hardly ever see Nellie Olsen or Hamlet in Star Wars fan literature.