interestingly, when two people are pedaling the power loss was slightly less compared to captain only pedaling. It was around 4.5-6%...this was not statistically significant (likely due to the small sample size of the preliminary study), but it was a consistent pattern.

from our paper:
When the captain position is occupied and the stoker position vacant, power values for the captain were approximately 6-7 percent more than those recorded at the rear hub. This difference in power is greater than the 2-4 percent that has been reported previously on single bicycles due to mechanical loss in the chain drive system (Gardner et al., 2004; Kyle, 1994; Kyle & Caiozzo, 1986). The greater length of the drive train for a tandem compared to a single bike may explain the increased power variation during tandem cycling. However, overall differences in the accuracy of the SRM and PT systems may also have contributed to this difference (Gardner et al., 2004).