I have to ask... how are you feeding your turtles? Any type of aquatic turtle needs to be in water to eat and digest properly. The sneezing is probably the start of a respiratory infection due to being too cold. I am sure you are trying to do the best for them that you can, but if you cannot get them into an aquarium (about 78degree water) of some sort with a way for them to bask (about 90 degree area with a uva light) to dry off and stay warm enough then you are essentially keeping a fish out of water. They may live longer, but they do not have what they need and will develop shell problems among other things.
I'm not trying to be mean or harsh, but from what little I've read from your posts it sounds like you are not providing the things they need for a healthy long life. Turtles live up to 20+ years by the way.
