Ok, so this may be obvious, but separate the two turtles now! If the sick one has something contagious, then you don't want the other one getting sick. Unfortunately, there may not be too much you can do for the sick one, but you can try to make it comfortable. If he's weak, then lower the water level so that he can touch the bottom and still breath at the same time. Raise the water temp to 79-80 degrees. It doesn't sounds good, but keep us posted and good luck!