How does this power supply (resistor/capacitor) divider filter work?
The intent of the resistor voltage divider is to ensure that the 450 volts is divided (approximately) equally between the two capacitors.  

Perhaps capacitors rated at more than 450 volts were not available, so two lower-voltage capacitors were used in series.  Due to the variation of the capacitor's values, and leakage current in the cvapacitors, the applied voltage likely won't be split evenly between the capacitors if the resistor voltage divider was omitted.