The reason the one may not be basking is there is no temperature difference between the basking area and the water. I saw no mention of your having a UVB light for them. They need the UVB rays in order to make full use of the calcium they need for strong bones and shell.
You can use a regular light bulb in a clamp fixture to provide heat for the basking area. It is very typical for turtles to head back to the water when some approaches while they are basking.. remember usually they are the prey

You may wish to get them a basking dock or make one so that they can both get fully out of the water to dry off.
You can give them a piece of cuttlebone to chew on instead of that calcium turtle. I've not read/heard any good things about that sort of thing as far as turtles are concerned.
Is there a reason you only have the tank 1/4 full? I have a 75 gallon tank also with 2 turtles and mine is filled as far as I can without the turtles being able to escape. They LOVE to swim
As for RI... it's a respitory infection. It can lead to pnemonia and death. There are any number of symptoms. You can read more about it here:
http://www.redearslider.com/index_health_1.html
Sorry this got to be so long. If I've not said welcome to the forum before now.. Welcome
