They have also, wisely avoiding much of the media, enlisted a lawyer to speak on their behalf on the matter, and it was the lawyers filing that led to the investigation being reopened again.
For a pretty complete chronology, see this page on the Swedish prosecution authority: https://www.aklagare.se/en/news-and-press/media/the-assange-...
"We are going to do everything we possibly can to get the swedish police investigation re-opened so that Assange can be extradited to Sweden and prosecuted for rape. No rape victim should have to wait 9 years to see justice be served." https://twitter.com/ElisabethMFritz/status/11162929511251804...
In many jurisdictions prosecutors don't need the permission, nor cooperation, of the victims to bring a case to court. This is especially important in crimes of sexual violence because the fear of retribution causes many victims not to report; and because some victims will be being coercively controlled by their partners.
In less serious cases the victim's wishes are taken into account. However, the prosecutor is the one who charges the accused, and consent or cooperation from the victim isn't needed. It is hard to convict in cases where an uncooperative victim is the only witness, but it happens in domestic abuse cases all the time.
As far as I know, and at least Assange has claimed this publicly in a video at least once, these two women themselves did not make rape charges against Assange.
The case was picked up by some prosecutor who initiated the charges.