id be careful adding a new turtle, make sure the meeting is on neutural ground and watch every move for many hours after to be sure there is no agression and watch again when you add them to the tank. I would wait till the little one is at least 4 inches as I think someone said before.
Be prepared you may need to keep them seperate if anything goes bad
F-in I would not introduce them at all. The big one my consider the small one as food. It will be quick when the larger decides to take a bite. I suggest not to do it. m.