You can continue keeping the two baby water turtles in the tank for a little while, because they're still small. However, if they grow bigger, you have to get a bigger gallon. The rule of thumb is a turtle that has a carapace length of 8 inches needs at least a 20 gallon aquarium to itself.
I have a 55 gallon I got cheap from a friend, and before I was thinking about buying a 15 or 20 gallon for two babies. Every website will tell you you need 10 gallons per inch of turtle but I find this not to be true, with proper filtering, care, basking light, and attention your turtles should have an ideal environment in your 10 gallon :). (once it becomes a tight turn for the turtles to turn in the tank, you should consider a bigger tank)