I've done some riding while wearing a FitBit. The FitBit is worn on the wrist and uses an accelerometer to track movements. It's fairly accurate for tracking running and walking. Because my upper body doesn't move as much, the FitBit tends to underestimate my activity level when riding on a single. However, riding on a tandem I get much higher activity scores compared to covering the same distance on a single. My conclusion is that my upper body is doing a lot more work keeping the tandem balanced (not surprising when the stoker is 8 years old, but I think it's true even with an experienced stoker).

My wife and I once took the kids to school on our tandems. We discovered that we were faster coming home without our stokers. That's probably also not true with a good stoker, but explained a lot to us about why tandem rides feel like such a workout.