Two isolated chokes only work in differential mode, while the Cm choke raises the CM impedance which is essential due to the imbalanced differentical impedance of both a switched load to ground and transient line impulses.
But CM impedance creates a high Z series path , which with shunt caps creates also a differential LPF . This is for attenutating large CM noise internal AND external but with the two shunt caps to ground creates a high a Z attenuation ratio , resulting in differential reduction of noise.
So the two coils MUST be tightly coupled and large in value (mH) for low frequency noise 50/60Hz line. For even better line filters low value CM (10uH) choke is cascaded in series for higher switching noise attenuation back to conducted line noise required for CE and FCC qualification.