Offline switcher SMPS design with two phase inputs
It seems your question comes down to how to get rectified DC from two AC voltages with a common neutral.  Here is the general case way:



The resulting DC is not referenced to the incoming neutral or any phase.  Both DC+ and DC- should be considered live and dangerous.  However, if this is the front end of a power supply, then the result will be chopped to drive a transformer anyway.  That provides the isolation, and allows you to connect any one point of the output to ground.

The rectified DC voltage is the instantaneous maximum difference between any of the three input lines.  With a single phase connected, or both AC1 and AC1 in phase, that will be the AC peak voltage.  With AC1 and AC2 out of phase, that will be the peak to peak voltage.