Writing a psychological thriller with unnamed characters

I'm writing a psychology thriller featuring two female characters who remain unnamed for the first few scenes. They're extremely close, but revealing their names too early would spoil a significant plot point.

My question is how do I handle dialogue and action lines without using their names? I have a narrator, which might help, but I'm unsure about the best approach. This unnamed phase will only last a in the first Act, until one character is addressed in a later scene. Any advice or suggestions would be greatly appreciated!