So far, I think it would be a good aproach to split the workflow into different states of the worldbuilding process.
First state would be like just blocking out one part of the world, then go to the next one, while another person does the details, texturing etc. on the first part....but when it comes to more than 2 or 3 people working on the world, this becomes much more difficult and chaotic.
When you are concepting your world first and sketch it out, maybe its possible to mark the different parts of the terrain with different textures and every member of the world building team just works within his marked territory. But you have to be very sure about the dimensions of the different parts of the world, which is really difficult I think. You don't want the player to have too long ways to walk:(
But all this thoughts are not satisfying solutions at all

Any more ideas?