my FIRST piece of advice would be to SEPARATE the turtles! if they are already tearing into one another, it will only get worse! second of all... it sounds to me like you are having either an 'attitude' problem or a territorial problem with the turtles.
what size are they?
how big of a tank are they in?
If the turtles are too big for their tank, they become territorial and this could cause the one turtle to snap at the other one.
did you ever think that the one turtle who bit the other one is a bully? i mean maybe the one turtle stays on the dock because he's tired of being harassed by the other one!
either way i definitely think that you should separate them.
do you see any apparent damage on the turtle's leg? if so, i would say to take the turtle to a vet. They could probably give you some kind of meds to put on the cut!
good luck! 
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~ Evie 
Pets:
RIP 0.1.0 German Shepherd {Dusty}
0.1.0 German Shepherd/Collie mix {Shadoe}
0.0.2 RES turtles {Booger & Snot}
1.0.0 African Clawed Frog {Bingo}
0.0.2 Snail {Gary & Larry}