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You would use genitive (žen), if the numeral was higher than 4 (ie. 5+ or indefinite numeral). From the amount of 5, the things/people/etc. are no longer viewed individually, but as one group (due to historical reasons).
jedna žena přišla (singular); dvě/tři/čtyři ženy přišly (plural); pět žen přišlo (again singular and the noun-subject is in genitive plural).
I'm quite sure it's mentioned in the tips and notes as well if you want to read more about it.
To me as a native American English speaker, the only possibility of two people loosing their head would be co-joined twins with only one head. If we think of the heads of companies there are slightly less unlikely possibilities. Two workers could work for two different companies headed by the same person. If that head person left both companies, the two workers would loose their head. If the companies had different heads who left at the same time, the two workers would loose their heads.