Ah, okay. So just to make sure I've got it correctly -
If we both want the choice of playing on either system, we would each need a copy for both PC and PS4 of our own. With our respective subscriptions, we could link our PC copy to our PS4 copy's registration code, so we could log on to our account from either system. Meaning, we would need (in total) two copies of the PC version and two copies of the PS4 version, one subscription for me, and one subscription for her in order to be able to switch between the platforms at will.
So lucky - the game is 50% off on Steam today! The internet is begging us to start! Haha =P