Use two 250V series cap in APFC instead of single 400V
If 2 x 470uF  capacitors are connected in series, the result is a 235uF  capacitor, but only if both are identical the maximal voltage of each one will be exactly 50%. If the capacitance is not exactly the same, the voltage will be unbalanced and could exceed 250V for one of them. 
Furthermore, for SMPS, the filter capacitors need to be suited for the high current peaks and resulting dissipated heat from the series equivalent  resistor. Often parallel capacitors are used in those devices, but never series capacitors.
In other words: those 2 capacitors as substitute most likely would not be an improvement.

But to get the right capacitor, the next recycling yard may have some old TVs, PCs, monitors or other devices with SMPS, where some 400V capacitors could be salvaged for free. Caveat: A few years back, those capacitors were prone for failing, testing before use is essential.