Can't speak to a DaVinci, which I think would be a totally different dynamic. From my experience on a "standard" tandem it's just a matter of practice. Actually, as captain, you can help her learn to spin faster.
When we started riding together, Pam couldn't comfortably spin at anything above 90 RPM. I figured out pretty quickly that at around 92, she would call for a shift, and plotted my strategy around that. What I would do would be to run up the cadence to the 90-92 rmp range and try to hold it a while. If she didn't call for the shift, I would inch it up a little higher for just a moment, then shift. Over time, I figured out that I could hold the higher RPM's for longer periods of time, and eventually was able to push my shift point higher and higher.
After a few years of riding now, we comfortably spin along at 110-115. Anything over 115, and I'm ready to shift!