CliffordK, Thanks for mentioning helicoiling a RS crank. Honestly, I'd considered whether it's possible to use a RS crank, and simply tighten it up a bit, or even use blue Loctite. I've never seen helicoils for LH threads for pedals, but, I trust they exist. Seems like a good strategy. My wife's cadence is similar to mine, and that hasn't been an issue. Stoker crank is a 170mm, which is what's on her single bikes, too.
bwebel, You've confirmed my suspicion that what I want is rare. It almost seems that 175mm cranks are standard on all non-custom tandems. Loctite is a brilliant suggestion, at least it's an inexpensive way to try it! Thanks for your reply.
Carbonfiberboy, While I thank you for reading and replying, the message feels like what I'm trying to resolve isn't legitimate. I've ridden numerous bikes, and have over 80K miles of riding bikes in last 7 years. While I'm not much of a bike mechanic, I'm a fair rider. I'm happy you don't notice the difference of 5mm with your cranks, but I do. Why else would I trouble you all to post about this? It really bothers my hips when I stand to pedal, and my knees know the difference, too. I have the captain's cockpit set to same dimensions as my road bikes, as well. I'm going to pursue the 170mm cranks, and if it doesn't resolve my issues, I'll be writing to you to let you know....