I am not a fan of using a phone as a cycling computer but this app may have some utility for those yearning for dual tandem power meters. Designed for power meter testing, it can display multiple power meters on the same display at the same time. It was written for a very small audience of testers so the author must be very responsive to requests.

As it is the stoker has to do a little mental math to see their power or subtract the files offline for analysis. It seems to me the author could adjust the app do the math and display display the captain's power from one meter and compute the stokers from the rear power meter less the captain's power meter.