If it is adults, then when they bite, the other turtle should not "get hurt".. As in, dodging or something. If they were doing the "mating bites" the turtle being bitten shouldn't really mind. If it is younger turtles, the same thing applies.
The fluttering thing could also be a territorial thing. Is you tank large enough for both of them? When they stare, does one of them tend to get a bit scared and back off a bit?
I have young turtles, so if someone has an adult turtle and knows more about them, please correct me if I am wrong.
:You know when the turtles do that weird fluttering hand thing? Why does the guy one try to bite the girl when hes doing it? Is it normal? Is it Harmful? Should I isolate the guy one?