AIUI (but please correct me if I'm wrong), you can't. At least not without throwing more hardware at it. Otherwise, if you try to run two enwiki dumps concurrently on the same hardware, you'll find that they both finish in _twelve_ weeks instead of six.
If not, let's just run _all_ the dumps in parallel simultaneously, and the problem is solved! ...right?
We're already running multiple threads, I can't see why you can't have more than one of them doing the same project. We could run them all in parallel if you want to donate enough servers to have one thread per project.