I myself keep a few diffrent species of both freshwater stingray and softshell turtle. But have never kept both together although they require simular housing the turtles require a few diffrent things.
It's best to be avoided in my personal opinion. Both of these animals are very curious by nature and this can lead to problems causing seriouse injuries on both sides. You will always have people that get way with it and say it's no problem however. It only takes a second for things to take a turn for the worst maybe not today, tomorrow or next week but one day.