they dont need company, 2 turtles would just be more food, filtration and tank space needed. a 20 gallon for a small one would be fine, even two babies, but they WILL grow, and if you follow our advice and care for them properly, they will grow fast.
get some gravel and slope it up to make a land area.
they like to eat crayfish ,worms ,shrimp of all kinds(thawed frozen, dried), turtle pellets, crickets, june beetles, small fish, etc
get a suitable filter, they are messy eaters and make lots of waste.
they also neep a lamp that gives off light that mimics sunlight, reptile bulbs are dont cost that much and worked for me. just read about turtles, there is plenty of info.
hope you enjoy your turtle!