When I in cafe with Melody and Amy, I fail. I need a save file.
That means you were still going out with both of them by that point.
You would need to go back far enough so that one of their paths could be closed before that encounter - maybe check your local saves to see if that could be done easily enough?
To sort of cheat, you could just go back before the restaurant scene and turn OFF your relationship with one of them from the Melody game settings (click upper right corner). That doesn't allow you to experience more of the prior game with one of them cut off, of course.