Is it possible that one tartaruga can eat another after living with each other since they were babies?

I went to an amusement park with my parents, and while we were gone the rest of my family got two baby turtles. After I came back, I saw the turtles and soon I fell in amor with them. I loved to pick them up, but I wouldn't be able to do that after a couple years. After one month, I figured out a bad fact: In 15 years, Zill was going to grow as big as a jantar plate, but Alex would stay the same size forever. So now, I'm worried that in the future, Zill might eat Alex.