There are two different ones. Not sure if you have to trigger one before the other, but if you do, Emily's bed is the older one, so the one you should start with. Basically you got to corrupt them a lot.
In Emily's bed, you will start getting the option to make Emily cum. Do that until it doesn't offer that choice any more (like twice), Frisha will then sneak in after you go to sleep. Change speed and position until Emily wakes up. If Emily doesn't wake up, she isn't corrupted enough.
In Frisha's bed, make sure you have fucked her in the ass enough to get her wearing the better dress. Fuck her before bed enough days until she asks for a second round. That second round will bring Emily into the room. That's all that is needed for that.