15' X 15' X 36" is roughly 5042 gallons of water. L X W X D = Cubic feet X 7.47 = Gallons
Space for them to avoid each other is your hiding spots..
Unless one is an Apex Predator.. LOL^ agreed.
It isn't like fish where the attitude is "if I can see it it must go away!". As long as the turtles get their own personal space and don't feel crowded they should be perfectly fine.