Each car has kinetic energy KE=1/2MV2. Since they are equal and opposite the cars come to a stop at the point of impact over the distance of their respective crumple zones.
With a car hitting an immovable, undeformable wall, it comes to a complete stop at the point of impact over the distance of its crumple zone.
If the speed prior to impact was doubled, the KE ends up being four times as large due to the V2 term.