wow.. i dont even know where to start with this one... first off, if your turtles are 4-5 years old, this means they must be about 4-5 inches long... they should DEFINITELY be in a MUCH LARGER tank than a 10 gallon. is that even enough room for them to move around in?? mine are in like a 40 gallon tank, and they are only like 1.5 inches each! the 'general rule of thumb' is 10 gallons of water PER INCH of turtle... and although most ppl dont follow this, it should be CLOSE to this regulation!
second of all, the best thing you could do for your turtles is to first get a new tank so that they have room to move around, but second of all, if the biting gets worse, separate them! you wouldnt want one of your turtles to end up getting severly hurt by the other. this would mean vet costs, which would be A LOT more than just a simple 50 gallon tub from wal mart.
but as a side note, they could be mating. im not THAT sure of the mating rituals because my turtles are so young yet... but im sure someone else could answer that for you!
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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}