I kinda managed to solve this problem, you have to start rebecca's route, but when it tells you at the missions to continue it's route, do not open your phone inside your room, and even try to open it only at night, this way you can go and work with her, but you don't have to cancel any of both routes, because when you finish Becca's route you cannot get money any other way, at least, that's how i managed to get it