Can you put a male turtle and a female turtle in the same tank without them mating?

This may sound like a stupid question, but I'm very new to owning turtles and I'd like to get another RES for my RES baby to have company. The problem is you can't tell the gender of turtles when they're little, but there are places that have gender specific turtle babies so I don't want to get a male slider if the one I have is a female.