Hi, It depends on the size of your set up and the size of your turtles. I've always been taught if you already have a set up and you are introducing another one, make sure the new one is bigger so it doesnt get bullied by the older one, but you would have to be careful on this depending on the sex. Turtles can be very territorial and can and will fight and hurt each other with the loss of limbs etc especially if they are male BUT if you have a large set up they can live together. If you have a male and a female then they will probably mate so you will have to take this into consideration. A male has been known to take a fancy to a rock if it thinks its female so it will most definately try mating with a female, then you have to think about providing some way for the female to lay her eggs,(whether they are fertilised or not) otherwise she may become egg bound and that will cause alot of problems maybe even death!!! Turtles are very solitary so they dont have to be kept with others. Hope the above helps although Im no expert just what I've picked up from keeping my two :0)