The story goes something like this- they in fact have the same procedural generator. Both work upon the same key- thus mountains and everything is in the same place, provided they use the same hardware or the code is protected against floating point deviations.

The problem is- where do you begin and end with that? If a player can modify a world, basically, every change has to be distributed.. you have a sort of highres minecraft on your hands. Without originally being intended to be this. How do you sync it, if multiple players join and merge a universe?

I would have thought something along the lines of an acyclic graph ala git.