Why not the Polar chain-pickup system? That's what I'm thinking about. One pickup for the timing chain to get captain's power, another for the chainstay to get combined power off the stoker's HRM. Subtract for stoker power. The Polar recording HRMs move from bike to bike, so all you need is multiple pickups. Riders I know using the Polar power setup say they work +- about 5% compared with SRM.