Well, just imagine with the FAN9672 dual interleaved PFC chip, you have two low side sense resistors needing to be routed back to the same control chip …..how do you properly do that?…its not really possible, because the ground terminal of each sense resistor, and the ground terminal of the FAN9672 need to be the ‘same’ ground place, …and that will be very difficult for the PCB layout. And when you think of the switching currents going through both of those sense resistors..its asking for trouble with noise…why not just have one pfc stage working for 10ms, then off, then the other one starts up, which means that even though there are two PFC stages, only one is ever working at any time…surely this is more likely to result in less noisy operation.?