As long as we're talking relative runner lengths of primary and
secondary runners, I started wondering about having two runners
per rotor.  I've never been clear about why there are two, and
wondered if perhaps it had something to do with running up and
down through the gears (which airplanes do not do).

Could someone provide me with a quick and dirty explanation of
why we have primary and secondary runners, and why we need them
both for steady state operation?  Seems as if DIE would also be
greatly simplified if there were only one runner per rotor.