My wife and I had this problem 20+ years ago. There is a solution. We got a counterpoint tandem where she has a recumbent seat in front and a freewheel and controls a five speed cluster. She usually picks a cadence 10% or 20% lower then the one I am using. I steer from the rear in an upright position. Yes, if she is pedalling my pedals move. I can live with that. She can coast whenever needed.
www.bilenky.com still makes them. They call them Viewpoints. Check the photo gallery.