Separating them was the best move as it sounds as though trouble was brewing between them and Chinese hamsters are solitary even though some members have managed to keep pairs, but it`s not common. You would know if one was a male just by the very visible scrotal sack, as you can see from Esmy`s image!
Females don`t `bulge` at the rear end like this so much. They are fab little hamsters (although I like to think of them as mice without long tails!) x