my electrotechnik mind is not happy with this arrangement

there must surely be a voltage regulator in this picture somewhere

PSU is putting out some amount of power, meaning volts and amps of current

making a failover for power systems usually requires an accumulator somewhere in the picture, ie, transformer and capacitors

adapting to the flow usually takes logic and gates

i'm not sure you have got this

unless you have some kind of large capacitor buffer in the circuit