It wouldn't be that hard to have each mapper do their own thing, however the map has to be designed in such a way right from the start.
Say you have some sort of abandoned space station (mapper 1's design), and a ship has crashed into it bringing the humans (mapper 2's design), and just for the hell of it, some sort of meteor, or asteroid has crashed into this station as well (mapper 3's design) which has brought the aliens to the map. Now each mapper can do their own thing, as long as they conform to the same scale, and the same design where the separate pieces are supposed to fit together. To include more mappers we could say the space station is a collaboration between many different factions, and these factions have come together for some humanitarian or scientific reason or something along those lines.
Now, I'm not saying this would be easy to do, or that it is doable at all, but it is an idea. Also, if a mapper fails to finish their piece of the project on time, another could easily take over, and the design can be changed as needed.
Thoughts?