The simple fix to that is have the car hitting another, stationary car at 100mph. It's still a horrendous collision despite the 2x crumple zones.

In reality yes, but I assume the thought experiment is meant to take place in Spherical Cow Land where all colliding objects can be considered the same. I'm also not sure how relevant the crumple zones were when this was first thought up, as cars haven't always had them.

both are crumpling, but the line between the cars does not change and acts as a wall.

Imagine compressing two springs in a line vs one against a wall.