We started racing our tandem in 2007 in local time trials. We used HR as an indicator of effort. We had no problem raising our heart rates to threshold level or slightly above. Typically, we climb at 70 to 80 rpm and 90 to 95 rpm on the flats. We ride OOP stoker leading.
We did a couple of state championship 40K TTs on the tandem on flat terrain and had no problem establishing and maintaining threshold heart rates. And we did one 40K ITT in phase and it was okay but we prefer OOP.
Some riders can push themselves hard and some cannot. I found that my individual time trial performance improved when I got my power meter. I found that I did not push hard enough on the slight down hill. So even on single road bikes, it is harder to keep power up on flat to rolling terrain than climbing but it can be done and requires training and concentration.
Also, low cadence power intervals do not raise the heart rate as high as 10% higher cadence intervals at the same power. So know that if you are climbing and the cadence is 50ish the HR is going to be lower. What happens is you will use more strength and different muscle fibers and more than likely fatigue the legs sooner.
Riding a tandem is similar to dancing with someone. One can be a great dancer but when two great dancers try to dance as one, it takes practice to move with efficiency, grace, power and speed with the proper steps in time with the music. Stepping on toes is a no no. However, I think that some tandem teams step on each others toes or both try to lead. YMMV.