The Arrow/Flash crossover (which was just one from the many that will follow concerning these two television series) has proved to be a success.
As far as Flash is concerned, the numbers said that only the pilot was more successful. As far as Arrow is concerned, although the show is one of the best superhero shows of the moments, the episode blew all the ratings.
This being said, you have also heard that Supergirl is intended to be told in the same universe as Flash and Arrow, so this means that more than one crossover will be made between these shows.

But then again, are these three television series the only ones which could share the same universe? Is it possible to see even more crossovers in the future?
Green Arrow, Flash Crossovers

While it was said that the TV and Cinematic universes will be distinct, it is very well known that these two universes will be a part of the same multiverse. So, in a way, it will be pretty much like in the comics.
So one possible crossover will be the special appearance of Green Arrow in one of the future DC big screen films "“ but what about Flash? Well, we already have a Flash in the TV series, as well as an announced Flash for the big screen (with Ezra Miller playing the title role).

