Welcome!
Like people, some RES are slower-growing than others. There's nothing to worry about, as long as there's nothing else wrong with their shell, skin, behavior, etc. What is their diet and lighting like?
With all reptiles, RES especially, it's ideal to do the fewest amount of tank upgrades--otherwise, you'll be buying new enclosures and filters all the time! This is especially problematic, if your either of your turtles end up exhibiting aggressive or otherwise competitive behaviors with regard to the other one. That being said, you may want to get 2 separate enclosures now. Guess how I found out...
That being said, I would recommend purchasing a large (at least 55 gallon) aquarium (maybe 2, finances permitting) at Petco's dollar-per-gallon sale. If you're really a smart shopper, though, I heartily recommend Rubbermaid stock tanks. Lots (if not most, at this point) turtle enthusiasts use them--just be sure to somehow stop the drain plug from leaking.
http://www.rubbermaidcommercial.com/rcp ... tock_tanksRemember, Craigslist is your friend!