It's hard to say how long any tank will last any turtle because depending on sex of the turtle (females grow much larger and faster), breed of the turtle and feeding (feeding alot of proteins will cause the turtle to grow larger faster also) each turtle grows at different rates.
The general rule for a turtle is 10 gallons per shell length. Your 38 gallon will be good til the turtle is a little over 3 inches. It's illegal to sell turtles in pet stores in the US until they are 4" shell length, so it seems your tank will be a wee bit to small to start with.
Sand is also an awesome substrate for a turtle, they like to play and dig in it and even munch on it a bit. I'd never go without sand in another turtle tank i have, mine just loves it to much to take away from her.
You might also consider just getting 1 turtle instead of 2. I bought 2 at once thinking the one would be lonely if i only got 1, but ended up having to return one of the turtles a week later because they fought. Turtles are pretty solitary animals, and once aggression starts towards eachother it wont stop and the only way to fix it is to permanently separate them. It's cute to see them as a pair but it doesn't work out very often that way. My little girl is so much happier alone.
It's great to see you doing research before you bought them, not a lot of people do. If you have any other questions feel free to ask. And welcome to the turtle community
