My two turtles grow unevenly

I am very much speaking theoretically here, as my practical experience with chelonians is limited, but, mealworms are very poor nutritionally. Try to mix it up with some other insects, maybe the phoenix worms or blood worms. Or some small feeder fish left in there, like some guppies. If you let some guppies swim around in there, it is more likely they will both get an opportunity to feed.

Let me know how it goes.