Thanks for those details. Great that you are out there competing.
I haven't had individual power meters on the tandem but know from my single bike my ftp is around 260-270w. We have done a few 10mile tts on the tandem with a power tap wheel and usually average 420w. That gets us a 26mph average on a flat and smooth out and back course. We are 300lb.
That discrepancy between power readings on single and tandem is strange.
It would be interesting to run a power tap wheel as well and see how it all adds up.