What type of softshells are they?
I would keep that DBT in a sepperate tank just so he doesn't scratch the softshells capraces (they get infected really easy). And DBT would probably prefer a little more of an estuary type habitat. They like salt in their water. Depending on your softshell species, they might like salt too.
Depending on tank size you could keep both softshell turtles in the same tank, but if it is small you might have problems. I have kept softshells together many times and only once had a problem when intoducing a second male in with my two Florida softshells. My female tried to kill it (it was nasty).
I could be way off, but when I had a lot young softshells, the sick ones would always be basking or barried in the sand. Most of them would have respitory problems and usually got well on their own with good living conditions(clean water, good food, and fresh air).
Another thing to mention is that softshells like to burry themselfs in sand, but it is not always needed. I used to use sand, but I always had water quality issues. Now I use small stones (pea gravel) and I don't have to worry about bad areas in the substrate. I just gravel vac all the time. You can use sand, but I have found it to be more trouble than it is worth, for the animal. For my big softshells I use no substrate. Seeing as how my female Florida is about 5" tall(plastron to caprace), having 5" of sand or gravel would be trapping so much debris that it would always foul the water. I find that if they can hide under something or feel that they can't be grabbed all the time(big enclosures) they act fine. I never have the "diggin into nothing issue" that you see with softshells small tanks.
