If she wants a bigger gear, then you should probably give her a bigger gear and then you can pedal with the effort that you would like to put out. Once she gets tired of pushing the two of you around, you can pick your gear and pedal the way you would like. Though you will probably be putting in more effort to spin on the tandem than you would on your single because you are also pushing her feet around in circles. You could also think of the low cadence, high resistance work as power training. You can ride the tandem to increase your power and then ride your single to work on your spin.
When we started riding together our prefered cadences were quite different. Kayla, a non-cyclist, was comfortable up to the mid to upper 80's and I usually pedaled at 95-105. Initially I pedaled slower and together we worked to increase her comfortable cadence. Explaining the load on her knees by grinding out a big gear she found a good reason to pedal faster. One thing that we discovered to make the higher cadence more comfortable was to switch from a suspension seatpost to a rigid post. Two weekends ago at STTR in Elmira, NY I watched other stokers on suspension seatposts bouncing while they pedaled. At this point in time we are comfortable pedaling in the 90's to 110's. I believe that the seatpost played a large role in this comfort.