Your heroines are thieves. You decide how they conduct their crimes. Maybe they’re bank robbers. Maybe they are modern day female Robin Hoods and they steal from the rich. (I like that idea. Maybe that’s how they start their career but one of them gets greedy and the lines between black and white start to grey. She starts stealing from people who don’t necessarily deserve it and her partner, your heroine, wants out.)
Decide how they met and why they became thieves. Then decide how a man gets between them. How do they fall apart and why?