I think you're assuming that both cars exist in a vacuum like space, completely independent of other forces. Surely both cars are part of a larger system and in comparison to the earths gravitational force the force they exert on each other is a non-factor.
Thanks for your comment. I agree that there are other forces at work on earth than in space. I believe there is still much to learn about gravity and it is just a few of my thoughts on it. I also need to mention that most scientists believe that outer space is not a true vacuum either .