Turtles will eat until they throw up, then continue to eat. And they're notorious beggars.
Don't give in!
As Oct said, feed a head sized amount of pellets each, every other day, and start introducing veggies. There's a good sticky at the top of this forum that has what veg you can feed (in addition to the nutrition page on the main website).
And don't forget your cuttlebone. Peel off the hard backing, and you can give it to them whole or in chunks.
Finally, make sure both of them are getting about the same amount of pellets. You want to make sure one of them isn't hogging all the food.