Multiple logging devices, how to zero PM?


I use TrainerRoad through an old iPad + ant+ dongle.
I also like to log with my edge 820 at the same time for backup/safety.

Both devices record HR, wahoo blue SC (for ‘true’ cadence) and powertap G3.

Which one should I use for zero setting G3 at the start of a ride?

If I do both, does it just recognise the last one?
I have the auto-zero cal enabled on 820, assuming it will correct any drift when coasting at the end of blocks. But TR app doesn’t have that feature, only calibrate.
If the communication is bi-directionnal when setting zero, which device is actually taking the lead? 820 or TR app? Will it actually use the auto-zero option of 820 if a calibration was done with TR at any point?

Sorry for bunch of questions but not sure how to do this right in a reproduceable way…

Reason I ask is have seen a huge difference in power to perceived effort in the last two rides, also clearly visible on HR. Like what seemed moderate hard is now quite extreme and I really suffer in sweet spot now, went from 20’ somewhat uncomfortable blocks to barely surviving through 7’ blocks (thank god last workouts were not massively long blocks…).

Looking back, I had a session where TR app was shutdown by lack of battery on the iPad, towards end of session. The trainer stayed apparently locked in the last ERG setting and I just finished the remaining block using 820 timer.
The following session TR seemed to struggle to lock on ERG target so did a manual TR calibration of G3. Since then, I’m living in a world of pain!
All the previous rides were fine and seemed to match the road feel, power-wise (auto zero on all the time), so looks like it has something to do with calibrating through TR app…

Any help appreciated!

Use any device. Only one is needed at the start of the workout.

Thanks, doing a zero cal before every ride now and seems consistent, didn’t realize auto-zero wasn’t reliable, especially on the trainer. Sneaked in a ramp test between workouts, which came out at 250w. Sticking to that, latest workouts feel much more reasonable (as in I’m not going to die but still a bit challenging), including o/u like Palissade.
Also did a static ‘calibration check’ with weights on the pedals that was within 0.5% of expected torque so looks all good.