It sounds like MBD (soft shell). Are these hatchlings or
older turtles? How long have you had them. This results
from improper feeding and no UVB source. Where did you get
them? breeder?

I got my baby as a tiny hatchling and have never had any
health issues with him and he is just over a year old.

Don't give up on the other one. Try to feed it some softened
Reptomin sticks and get him in the sunlight or a good UVB
source (make sure he has access to shade so he doesn't overheat)
Better yet find a reptile vet. You may be able to save him.

Linda