The DC TV universe is a very unique universe in that it resembles the multiverse described in the DC Comics books quite well. We’ve seen a world of Batman before he was ever the Dark Knight, and on the CW shows, we have alternate Earths that interact all the time.
In Flash alone, we’ve been to Earth One, Earth-2, and has a character from Earth-27 in HG Wells. But also, we have Supergirl, who is from Earth-19. By the laws of the Multiverse theory, there are incarnations of every DC Comics character on all these Earths. Yet, we haven’t seen any of the Arrow/Flash characters appear in Supergirl. Executive Producer Andrew Kreisberg reveals to IGN why this is:
“The problem with that is it feels to us like if we do that once, we’re going to open the floodgates of insanity,” Kreisberg. “It’s like Supergirl’s over here, and it’s its own show. Technically speaking, it’s just another Earth, the same way Earth-2 is or Earth-3 or Earth-19. So technically speaking, there should be an Alex Danvers on Earth-1, or Earth Prime. But if we start doing that, I just feel like… Not that all hell hasn’t broken loose already, but unless we had a really good story for it…”