You sound like you are entertaining the idea of getting another res. I did and found it to be a huge problem due to aggression. I now have two tanks for three turtles (two in 1 and one alone). Females are more aggressive than males. Basically they are solitary creatures with the exception to mating, and once that is over then back to normal. If you do decide to add another, be prepared for injuries and possible death. Having a second tank ready or one large enough to give them alot of room will be needed, only to hopefully prevent aggression. It may never happen but be ready if you do get another.
Males can be really aggressive too, especially with each other. Depends to some extent on the turtles, but the tendency is towards serious aggression for sure.