Re: Prometheus: Ridley Scott returns to Sci-Fi
Nothing to concretely tie them together, nothing to prevent it either.
There are a few elements from the Alien franchise in the Blade Runner movie, but those are more like inside jokes, added by the art department (similar to how there's a Millennium Falcon in Starship Troopers).
Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning.