I wouldn't recommend getting another turtle since there's a very good chance they won't get along when they get bigger and then you'll have to maintain two separate 120gal tanks. It can get very expensive. Also even if they didn't fight you would need a 200gal tank (or larger) to house the both of them as adults.
RES are solitary animals by nature, only coming together to compete for the best basking site. In captivity they can do fine by themselves. As long as you put some things in their tank to entertain them like a ping pong ball, a cup or tube to swim in/through (make sure they can't get stuck), an air bubbler that's set on a timer to come on several times a day, some feeder minnows, etc.
Edit: Never mind, I saw your post in the other section and see that you've already thought about this.
Posted: Tue Jun 19, 2007 9:48 pm 
