The saints' original names were the Latin
Augustinus and
Constantinus, not
Augustin or
Constantin. And
Augustine and
Constantine are not feminine names in Latin. In English the -us or -ius endings were sometimes removed from Latin names to anglicise them, e.g.
Marcus to
Mark,
Dominicus to
Dominic,
Vespasianus to
Vespasian,
Antonius to Ant(h)ony. I don't know exactly why the -e got tacked on to
Augustine and
Constantine, but probably because as an English speaker they look more complete and correct than
Augustin or
Constantin. Names generally don't end with -in in English. The only one I can think of right now is
Justin.