The idea that Mind is a world all of its own, non-physical and consisting of a reality equal with or superior to physical reality, originated with Plato in Greece around 500 B.C. There were no such theories around at the time of Zarathushtra for sure.
What is more interesting from a Zoroastrian perspective is to study the latest science of physics. In loop quantum theory - now the dominant theory in physics and cosmology following the demise of string theory - not even space or time themselves are fundamental. What is fundamental is instead nothing but causality itself.
Now if anything was extremely modern with Zarathushtra it was the concept that CAUSALITY is the fundament of existence (the stress on the importance of the ORDER of things). This is something we should focus on instead of trying to force a Platonian dualism into The Gathas where there is none.
2008/12/29 Special Kain
The whole point is that the question whether there are two or more worlds mentioned in the Gathas isn't as important as realising that they couldn't be essentially different from each other. There is only one law, asha, and asha governs all worlds - whether real, potential or merely imagined.
What we have here is functional monism as in pragmatism's notion of functionality replacing the absolute truth. The material world and all those fancy astral theme parks do not follow completely different logics!! They're not functionally different, so they're not essentially different, either. And since we're living in a post-ontological world, philosophically speaking, we don't have to worry about the question whether there are any heavens, because they wouldn't be much different from this world, anyway.
I hope I made myself clear enough, so I may finish in Rorty style, claiming that there has never been a problem whatsoever. :-))