Generally speaking, turtles are also one of the aggressive animals. If two turtles are kept together, it may cause two turtles to fight. If one turtle is bitten by another turtle carelessly, it will easily lead to wound infection and death.
Two turtles can't be kept together, and there will be fights between any animals, and turtles are no exception. Older turtles often bully smaller ones and sometimes even bite off their tails. Usually, it is enough to keep one of these animals. If you want to keep more, you'd better isolate them and don't meet each other at ordinary times.