First off, you may want to reconsider getting two turtles if you plan on keeping them inside. Two are not cheaper than one. If for any reason you have problems with aggression or other problems, you'll need 2 FULL setups. That's a tank large enough for each adult turtle, docks, lights, heaters, filters.. etc. Unless you plan on keeping them outside.
As for growth, you want a slow steady growth. How fast they grow depends on many factors such as, water quality/temp, UVB lighting and diet for starters. I suggest you do some serious reading on the main website and browse through the forum before you buy your turtles to make sure you have everything you need and as to be able to make a totally informed decision. I wish I had done that.. and there are a number of others who also bought theirs on impulse.
Keep us posted as to what you decide and when you have a chance.. pictures are always welcome