Multiple turtles in one tank

I would avoid having more than one turtle in the same tank, unless you have a pond that is of bigger portions so that if a turtle is getting harassed by another, it can actually swim away (vs. swimming in the same tank and being chased endlessly).

As far as the difference between male and female, do you know the exact species you are talking about? They have different visual cues to note if they are male vs. female.