In a 55 gallon you should limit yourself to only 1 RES. Dont get another turtle until you can double your tank size.
Here is my 120 gal which contains 1 RES, 1 River cooter, and some fish.

Here is the basking spot.

And here are ideas for how to make a canopy hood so you can fill the tank all the way up. So many people think they have a big enough tank with their little 10 gallons per inch formula, so they think their 4" turtle is happy in a 40 gallon tank when really they only have 15 gallons of water in it.
Basking turtles only need land to bask and lay eggs. If you are intent on breeding turtles you will need a large land area. Otherwise, a turtle only needs enough land to pull itself completely out of the water to bask. Dont waste half your tank on a basking area. The canopy hood is the best way to be able to fill your tank full of water. If your basking area is made by stacking rocks, thats gonna take up a lot of space in a full 55.
If you need help designing a canopy hood or basking spot or filter, I can give you basic ideas.
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