There is no fixed, guaranteed level of AC mains power so that the Tesla Model Y can always assume there is sufficient power to run the HVAC compressor, perform stator warming and run the low voltage systems.
Perhaps if there was a schematic. I can picture wiring in series, not in parallel. I don't believe Tesla would use two separate high voltage wiring harnesses for the AC compressor (one from the battery and the other from the PCS).
My understanding of how the Tesla Model Y is powered, charged (AC charging):
Mains Power (AC) => PCS (steps up voltage from 120V/240V to 400V, rectifies power to DC) => High Voltage Battery.
Mains Power (AC) => PCS => DC-to-DC Converter (This would enable powering some of the low voltage system, charging the low voltage battery directly from the Mains (my conjecture.) Note that this would not be applicable when Supercharging as there is no AC mains source.
High Voltage Battery => DC-to-DC Converter => Low Voltage system (powers low voltage systems and charges the low voltage battery as required.)
High Voltage Battery sends 360V+ DC power => HVAC Compressor
High Voltage Battery sends 360V+ DC power => Drive unit power control units (converts DC back to AC) => Front and rear drive units' motors for driving and power wave for stator warming.)