Why are my baby turtles fighting?

They are both in a 20 gallon tank .very spacious compared to their size one is like half of my hand (I have very small hands) and the other ones, the size of a quarter. But the other day I was feeding them and they separate container just because my filter has been acting up and I don't trust it to clean well enough specially food ...... and the big one was scaring spook the little one away when eating. a couple days later, I actually caught the big one trying to fight with the little one. (let me just add this is not like constant fighting it's just random and it does not happen all the time) but today I am feeling separate them in separate containers to eat and the little one literally bites the big ones feet. i got scared and separated them for a while. I have another 10 gallon tank, but it literally cracked the other day and I live in the middle of nowhere so I'm debating whether fixing the other one or getting those tanks separators.

Another question I had which I'm confused about. I swear these turtles are older than they look. Is it normal for one of them to grow faster than the other one? I mean when I got them, they were both the same size. I got the bigger ones. i mean when I got them, they were both the same size. I got the bigger one and it had its eyes shut. with a lot of natural sun and a water heater it got a lot better and started growing and growing ones it like woke up from being sick.....

And no stop telling ppl to go to the vet like if its not 150$ just for the visit minus literally everythinggg!!!!!!!!!! i need a way to stop them from fighting, and i dont even know whos male or female yet