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The thing about cross-overs, I think, is that they're very difficult to do well. Fanfic is full of insider references -- we can mention "the pheremone incident" or Nightfall, and everybody here knows what that means, but non-fans are likely to be confused. So one has to try to do a back-story that's comprehensive enough to help newbies figure out what's going on, while not being a boring re-hash for the established fans. And in a cross-over, you've got that problem doubled, especially the more detailed and complex the respective universes are.
Back before I became a Stargate fan, I happened to catch an episode that dealt with strong tensions between the Tok'ra and the Jaffa, with the Tauri thrown in the middle. See, the Jaffa are the ones with things on their foreheads, except for the guy who's part of the group from Earth, and the guys wearing boring brown are the *other* kind of alien, only one of the characters from Earth calls one of the aliens "Dad"... Huh?????

Once I caught up with all the eps on DVD it made perfect sense, but at first glance it was mystifying.
So, um, it's hard to write for two difference audiences at once. It can be done, and of course there's often overlap, with people familiar with both sides, but it's just tricky.
[/thread drift]
PJ