There are many thing to consider when putting two turtles together.
1] Is the new turtle Captive Bred? It should be if you are going to put it in with your established turtle. Otherwise you run the risk of infecting your old turtle with a disease/parsite.
2] Are they close to the same size? If not the bigger turtle will see the little turtle as FOOD! This is especially true if the original turtle is the bigger one, and you have been feeding him all this time. He just associates everything you drop in to be food.
3] Is the tank big enough? With 2 turtles you should have approximatly 10 gallons of water [not tank] for each inch of turtle. Thus if you have two 4in turtles they should be in a 50gallon tank with about 40 gallons of water. Please, do not keep turtles that big in a 20gallon tank half full of water. My wife and I had this situation before we moved them and the dominant male harassed the female to no end. She became very stressed and we had to act immediatly to change thier habitat or she probally would have died.
Then the last consideration is enviornment. As far as I know most RES, maps, and painted turtles are all tropical and native to SE United States and Central America depending on the specific species. So I think that you should be alright here. However, some maps and painted turtles do come from East Asia so either ask the breeder or do some research. A good place is your favorite bookstore!
Hope this helps, and I am sorry it took so long for someone to respond.
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Joshua M Guillemette
1.1.0 Children [Deven 10 & Hailey due June 1]
2.1.0 Cats [Micah, Judo, & Elizabeth]
1.0.0 Bunny [Bugs]
1.0.0 Red Belly Slider [Bernulli]
2.0.0 Yellow Belly Sliders [Maxwell & LaPlace]
0.0.2 Cumberlands [Pascal & Joule]
1.2.1 Red Ear Sliders [Kepler, Napier, Gauss, & Kelvin]