My wife/stoker and I don't have a great deal of overlap in our cadence comfort zones and I know many have correctly suggested exercises to increase stokers cadence. We may yet work on that, but I considered shorter stoker cranks. My LBS thought that given my wife's relatively long legs (and my relatively long legs) we might try longer captain's cranks.
I switched from 175mm to 180mm not quite knowing what to expect, but it's really made a big difference, dropping my range by 4 rpm or so. I would say prior to the change I didn't want to go much below 87 when working on the flats with a top of 95 or 96. I'm finding now that I'm comfortable working down to 83 with out getting into the "grunt" zone.
I'm absolutely comfortable with the slightly longer cranks, but everyone's mileage may vary as they say. Biggest surprise, as suggested above, was the disproportionate decrease in cadence compared to crank length, but then thinking about the geometry there is no reason to expect things to be linear.