Why are the two stories
(Jesus feeding 5,000–only counting men; 15,000 counting men, women and children, and
Jesus feeding 4,000)
in all four gospels for the former and
in Matthew and Mark in the latter?

They are in these places because they reference historical accounts of Prince, then King Izates, part of the Manu line of kings (think Em-Manu-el) feeding the victims of famine on multiple occasions with the help of his holy mother, Queen Helena.

Yes, in the famine of approximately AD 47; I say yes for these gospels were written after the famine.