IME low cadence = low HR. You just can't put out the watts at a low cadence. You'd be slightly faster with lower gearing and get better training. I'm sure it's possible to train so as to put at more watts at low cadences, but few of us have any reason to do so.
Also IME the way to get stronger on the tandem is distance. Something like 80 miles with 5000' of climbing every summer weekend will stimulate the system. The lowest hanging fruit is usually stoker strength. Most of us who have been riding for many years are doing well if we can get 5% stronger in any one season. There's usually more room for improvement in our stokers unless they are equally experienced. Also, at our age, we are fighting the drop-off. Every year we are potentially slower. So the faster an older team can ramp up, the more likely is success at any particular biking endeavor. It's now pass-riding season in the PNW. Next weekend we're doing Rainy Pass. The weekend after, Sunrise. Then the Paradise-Skate Creek loop. Weather permitting of course!
Come on up!