EMC filter in DC-DC converter cascade
I've never seen converter chip where required capacitance was specified as an exact number. It is always a range or at least a minimum. This goes for both input and output. So, in your case you have to figure out a number that a) satisfies both requirements from chip datasheets and b) sufficient for the ripple limit you set yourself.
Having said that, those large capacitors at the DC-DC inputs are usually decoupling, not EMC filters. The typical EMC filters for DC-DC converters are either LC or CLC and with much smaller capacitance.