Imagine a thought experiment in which the two cars and their contents are identical mirror images of each other.
In this perfectly symmetrical universe, after a 50 mph crash, the cars break up symmetrically: because the cars and contents are identical in every way, every fragment is emitted at exactly the same time from the same position and at the same speed, except for reflection.
Not only does every particle of one car collides with its corresponding particle from the other car, every collision happens in the plane of symmetry, with each pair of particles either stopping dead or bouncing away from each other.
But this is identical to the result of one car crashing at 50 mph into an immovable infinitely large wall where the plane of symmetry was.
Therefore, two identical cars at 50 mph each crashing is, in principle, the same as one car crashing into a wall at 50 mph — and different from a car crashing into the wall at 100 mph!