Hmm. Also late to the thread, but I've read every post. Count me in with the poster who wonders how accurate a powermeter can be under the conditions described in the o.p. Is it not possible that the Captain's display is showing an additional amount of power input by the Stokers cranks while the Stokers display is showing a corresponding loss? In any case, I think it is more likely the fact that the Stoker cannot select her own gears, rather than the fact that her cranks are of a different length that add to her frustration. In the recumbent world there are beasts called "back-to-back recumbent tandems" and these are mainly two wheel drive HPV's with the Captains cranks driving the front wheel and the Stokers cranks driving the rear wheel. Independently geared of necessity. It's where I would look if serious performance was an issue.
When I call for 'Power' to sprint through a stale green I don't always know if I got it all but if my Stoker does not hear a call of 'Stopping' (crank rotation) I know instantly that she did not hear it and I have to call again and/or fight the pedals if there must not be any acceleration at that time. Tandems were called an 'equalizer' by the o.p. and this is a common ... misconception, for want of a better word. Actually, in practice it is very hard to synchronize independent power sources. A single large engine has proven to be superior to two smaller engines every time, and automotive designers no longer experiment with them. Tandems allow couples to ride twogether. There is a trade-off of ultimate efficiency made for the greater good of ... twogetherness. If you read that as an indictment of heartrate monitors or powermeters onboard tandems ... well that's my take. The aero advantage of the excellent draft afforded the Stoker ultimately benefits both, and tandems are indisputably faster than singles, everything else being equal. But when is that ever the case?
I didn't give tandems much thought until 2005 when I met a blind woman who had ridden one a few times and was a member of a club that had tandems that could be rented. Our second date was in NYC's Central Park on a sunny Saturday in May and I was able to Captain the loaded tandem immediately. The brakes were ghastly. We never rented one of their bikes again and bought our own that very week. We have joined tandem clubs and met over two dozen teams in the last decade and more, none of which ride their tandems anymore. None have ever had a blind stoker. I think more teams expect the Stoker to be a laggard and this gives many Stokers a complex about how much or how much not they are working.
We don't worry about it. What would be the point? I know my Stoker is plenty strong. But if she wasn't ... we'd just have to deal with that and have fun anyway. We are slower than some, faster than some, but Team2024 always arrives wherever we are going at the same time. Finally, IMO crank length is an abstraction. It has only an oblique relationship to a riders height, inseam, femur length, etc. Simply put, cranks are limited by the cornering clearance available on road racing bicycles. Tandem bottom brackets are usually higher than road racer's and in theory Captains and Stokers alike could run 200mm cranks with no pedal strike problems through turns. In theory. Try to find 200mm cranks for sale. We have three tandems currently. On two of them the cranks are the same length for me and mine:170mm. I am 5'10". She is 5'6". On the exception, my cranks are 175mm and hers are 170. We have female friends that used to ride tandems at 4'11" and 5'2" and they too used 170mm on their tandems and now on their singles. FWIW.