Bit of a narrative challenge idea. The foundation is the ever classic hypnotism scenario where someone uses some method of hypnosis to get their foot in the door and implant some suggestion to start their mark on their way (like "we're lovers" or "you like it when I X") but the hypnotic state is a one-time thing. Either the mark develops an immunity, or there's limits to magic/science/whatever it is that let it happen in the first place, or the hypnotist simply forgets or otherwise loses the means of further conditioning the other girl. The original command is still in place, but the mark's personality is still otherwise intact so the hypnotist has to just use more mundane methods to further groom the other girl.

Really flexible on the setting and kinks in general. Hit me up if you want to do something with this. Can play either role in this scenario!