Why does some cookie recipes both put sugar and brown sugar? Also can we use sweetener like stevia instead of sugar in recipes?

Why does some cookie recipes both put sugar and brown sugar? What is sweeter regular sugar or brown sugar? Also can we use sweetener like stevia instead of sugar in recipes? Will it give a sweeter taste? If it works what amount would you put in if the recipe calls for 1 cup of sugar?

Thank you :)