I’m only an occasional tandem captain, but have noticed a few things compared to single riding:
1. The bike is heavier and takes a bit more steering effort. As a rock climber I consider my upper body to be in decent shape but my arms/hands are just a bit tired after a tandem
ride. This doesn’t happen after single rides. This is one way you might feel tired despite not having a higher power output/heart rate.
2. When accelerating I have an expectation for the rate I’ll speed up. If I don’t speed up at that rate I naturally push harder until I do. With a weaker (power per weight) I never make rate, so I always work a bit harder during those short bursts.
3. It is of course easier (power wise) to cruise on a flat. Weight doesn’t matter, power is increased more than drag, and no acceleration.
4. Also expected, harder up hills with a weaker stoker. Our combined team power per weight is lower than mine. I recently did a ride with a stoker who is stronger than me and everything was easier than on my single.
5. My general concentration level is higher because I can’t correct as quickly and because the consequences of crashing are higher with a stoker in my care.
I have spent some time riding solo on the bike and other than the slight weigh gain and longer wheelbase it’s not much different than a single.