why does one Terra seem to sink on one side of his shell???
This is a very bad sign, swimming/floating lopsided is a sure sign of respiratory infection (RI) which if left untreated will kill it, and probably the other one too as it's highly infectious.
You need to get the obviously sick one to a vet for a course of antibiotics asap, in the meantime seperate the two and sterilize the tank and all equipment. You'll need to set up a quarantine tank as it will be at least a few weeks before they can be put back together.
In the meantime, you can't sex them as hatchlings, depending on the species you may have to wait a few years, and you need to correctly ID the species before we can give you any advice regarding diet etc...
Just realised this thread is a week old, so it may already be too late?