Red ear sliders need a lot of room. The general rule of thumb is 1 inch of shell per 10 gallons of water (so a 4 inch turtle should have 40 gallons tank).
Think bigger and remember, if you have 2 turtles at 4 inches each, you need to make it 8 inches of shell. And LOTS of filtration. I would suggest a canister filter.
I have been suggesting a few different set ups to a friend of mine who has 3 turtles at 5 inches each. Either putting together one of those pre-formed ponds you can get at Home Depot. The 180 gallon one ran only $125. It would not get filled all the way and you would have to put in some floating/docking resting points, but its much cheaper than a tank of even 55 gallons new.
OR this waterland tub.
WATERLANDTUBS Warning, they are expensive but great for the animal.
