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.