Hermit crabs and turtles both share some common features. Both have exoskeletons i.e. Shells for their protection from external environment. Hermit crab is gregarious and social. Turtle is very peaceful animal. Hermit crabs like to live in moist environment. Turtles also survive well in water. So they can easily live together in a tank. Feed them with fresh fruits and vegetables because both of them are herbivores. Don't leave uneaten leaves and fruits for more than 24 hours.
You should keep them separated and if you do put them together watch them every other hour.
No. Bad idea. Please do not attempt to put them together.