Account Support

OK, I first bought the game for ps4 and i registered it to my square-enix account, then i purchased it on steam and before logging in (windows version) it me for the registration code, so it got linked to my pre-existing square-enix account. But now i want to be able to play on both systems and i was told i would need to add another service, so i tried that but it will not let me because the registration code was already used....i want to play on my pc too is there a way to add another subscription?