My understanding is that the dual recording is less to do with power meter accuracy and more to do with combating “saw-toothing”.
When on zwift you get a few seconds free power when you stop pedalling - a feature put in place so riders don’t notice short drops in signal.
E.g. if you were to pedal at 300w for 5 seconds, stop for 2 seconds then pedal at 300w for 5 seconds and repeat then zwift would record that as a constant 300w.
Dual recording catches this as the other device would record the pauses in power and the average would be significantly lower.