I have 2 hatchlings that are about 1.5". Within a month of getting them, I switched from a 5-gal, to a 10-gal, and finally to a 20-gal tank. I think 20-gal is a good size for two turtles that size for probably 1-2 years, depending on how fast they grow.
If you want to use a tube light for both heat and UVA/UVB, make sure that it says so on the package. Some UVA/UVB bulbs don't give off much heat.
As for water depth, you can fill it almost to the top, leaving enough space so that your turtles can't escape. When you have deep water, still make sure that basking spot is completely raised above water, and that there are some shallow underwater spots that your turtles can rest on. Just keep in mind that they breathe air with their lungs, just like us. So they definitely appreciate some spots in the tank where they don't have to swim to keep their heads above water.
Hope this helps!