Even though Prokris is the only daughter of Thespius to have been listed as having twins, the fact that Herakles had sex with only fourty-nine of the fifty daughters and yet had fifty-one sons, it means that one of the other daughters gave birth to a second child. It is impossible however to identify the parent since the name of the twin is not listed and neither is the name of the parent. Since Prokris was the oldest and bore twins, there are some versions of the story where the youngest daughter was the other to bare twins. Having the first and last of the daughters does make for a good tale but that is probably all that it is (because other than Prokris and the virgin daughter any of the daughters of Thespius could have been his mother.
Herakles & One of fourty-eight possible daughters of Thespius