Translation:The two populations are different.
Is there no way that "populations" could be translated as people? "populations" just makes it sound a bit academic, like your are talking about demography or statistics, and as such a bit useless to majority of people.
You probably are talking about demographics if you use this sentence. But that's okay, isn't it? Its an interesting topic sometimes.
The word 'both' is listed as a translation of 'les deux'. Voila 'both populations' should be allowed.