The picture is kind of blurry and I cant see what you are talking about, but his shell looks kind of weird..almost silver-ish like on one spot..does it smell? Is the area soft? (the shell and the "leg sore")
What you said about the one with the leg sore not swimming, is pretty strange too. RES are highly aquatic, they only come out of the water to bask or to lay eggs. Your turtles should be alert and skittish around you. When a aquatic turtle isnt being aquatic its a problem.
I highly suggest that you take those turtles to a vet (when one is sick, the other one can be too even if no symptoms are shown)..lethargy isnt a good sign and death often follows it
What is your setup like? Have you read all the info on this site on how to properly take care of these animals?