I'd not keep the other turtle in the tank for a few reasons, he's not getting what he needs because of the other's handicap. The water should be MUCH deeper for a normal RES. The gravel is too small... I understand these things for the blind turtle, but the other should have deep water. As TurtleSirens said, they should at this point already have a 20 gallon tank with the water at filled as high as you can without the chance of escape.
Also, do you feed them separately?