I think I've heard you can also play around with the crank lengths on a tandem.
Longer cranks tend to slow one's cadence down a bit, and shorter cranks can speed up the cadence, I think. So, perhaps try to give the Captain (that wants to spin fast) 180mm cranks, and give the stoker (who thinks the pace is too fast) 160mm or 165mm cranks.
It would be nice if you could get loaner parts from the local bike dealer. Or a "bike fitter".
Perhaps your bike manufacturer would help.
COMOTION is a local Eugene company. I haven't dealt with the directly, but perhaps they could supply the needed parts with the promise to buy them if you find the length you need.