pay if an accident accured. Wouldn't both insurance company have to pay me what the car is worth?

When I was 18 a women hit me and her insurance company only paid what the car was worth. I figured, if I could have the car insured by two different insurance companies my car would be paid for in its entirety. Is this illegal and will it be beneficial for me? I know that there is insurance that is called (gap insurance) that you can get when you purchase a car and it will pay the remaining balance on the car. However, I don't want to walk away with no car and no money for my investment if I get into a car accident.

What are your comments? Please, no opinions and no crazy comments. I appreciate all that answer and may the Lord Jesus Christ bless you indeed.