On Bluetooth, Favero Assioma Duo appears as two power meters in Zwift (L/R are separate), so the app only receives half the power data. At the same time, Zwift on Windows with an ANT+ dongle, TrainerRoad on a Galaxy S4 (built in ANT+) and Wahoo Element Bolt (ANT+) all report full power. Not entirely surprising because many L/R power meters report L/R separately over Bluetooth.
Seems to be the same problem as https://support.zwift.com/hc/en-us/community/posts/115019272943-Dual-pedal-power-meter-bluetooth-on-iOS-halves-power, which affects Favero, PowerTap and Watteam power meters on iOS.
Simultaneous might be another issue. I was able to login to Zwift on PC and Zwift on Android at the same time. That suggests I could record two rides at once–or someone (maybe several someones) could use the same account at the same time. – But fix the power meter thing first so I can test on the phone (discard ride) while running full power on the PC (save ride).