No, you would not get two full time salaries or double the pay if you worked just 40 hours a week.
An Interior Designer works with the interior structure, typically with an architect. During that time, an Interior Designer may pick out finishes for floors and walls etc, which crosses the line into Interior Decorating. An interior DECORATOR, has nothing to do with the structure of a room as their job is to pick out paint color, flooring, furniture, artwork, fabrics and lighting.
A Designer's job can easily cross over into parts of the Decorating. But a Decorator does not have the training to remove walls and re-design a structure without the education and knowledge about how load bearing walls, etc....
There is nothing stopping anyone from getting two degrees, but do not expect double the pay.