You should watch the movie once more, you have missed the whole thing.
the living partner was thinking that they were going to their home in one week, after every week they were replaced by a new set of clones exactly looking the same, so no new couples will rebel as every new set of clones will be thinking they are going home in 1 week.
in the so called radiation zone too the same set of pairs are cloned, and kept apart. probably these pairs are all over the earth, but never know each other nor met before cruise bumps into the other.