Originally Posted by wuther
From a comic book point of view they do not exist in different universes unless something has changed in the last several years.
This isn't a comic book, though; it's a TV show with its own continuity loosely based on that of the comic books.
Originally Posted by Westly-C
Uh...did you miss the episodes of Arrow that introduced Barry Allen this past season?
The episodes introduced Barry, yes, but they didn't introduce The Flash. Barry went into a coma after being struck by lightning, and that's the last we heard from him. Felicity never said, "Hey, guess what, guys? Barry woke up and now has superpowers!" I think there's still a difference between villains being given a strength serum that can be reversed and a superhero/villain gaining innate supernatural abilities that can't be taken away and never wear off.
Last I heard, the producers have already said that despite starting The Flash as a spinoff and having villains with superpowers in that show, they aren't going to change the "laws" of Arrow to have enemies shooting lightning bolts from their hands or fire from their eyes. We've been given to believe that Arrow will remain a show where Oliver fights "normal" enemies and other problems, and not supervillains.
Given that, I'm not sure how they can do crossover episodes without breaking Arrow's "rules", i.e. if The Flash needs help battling a supervillain, it would change the kind of foes Oliver faces, and if The Flash needs help solving a non-supervillain related problem, it wouldn't make much sense, since he will have already saved people from supervillains by the time the crossover takes place, thus making us wonder why he would need to ask for Oliver's help when he's already tackled bigger problems by himself.