My wife is having some shoulder pain in just her left shoulder. She also commented about hand pain, I believe towards the center of the palm/wrist. No lower back pain complaints. She stated that she thinks the pain starts to radiate inward and downward to the upper back as distance increases. This is common to both our new tandem and her single bike (a Trek Navigator).
Saddle position is the same on both bikes (height and rear offset). Reach to the bars seems the same. The bars on the tandem are about 2" higher than the saddle, and the bars on her Navigator are about 1 1/2 - 2" higher yet. For what it's worth, the bars on both bikes are on the wider side, and she has a mirror on the left side.
Thanks in advance for any advice you have.