Consider this: would you rather drive head-on into a parked car at 100mph or a brick wall?

obviously the parked car, but this is an entirely different question.

The parked car will move away, providing cushion.

When you have two cars in opposite directions, they cant both deflect.

You could secure the parked car so that it doesn't move, and its crumple zone will still slow the deceleration.

Nope. Consider the crumple zone of a trampoline, and the crumple zone of your knees. Now replace the trampoline with concrete.

Put your trampoline vertical have two people jump into each other.

The trampoline doesn't deflect, and acts exactly like a concrete wall.

The trampoline cant simultaneously deflect in opposite directions.

You're still failing to reason this through correctly. Let's stick to the central example, since the analogous situation didn't help you.

Same amount of momentum, two collisions. In one, there is one crumple zone, we will assume the brick wall is completely undamaged, for the sake of simplicity. In the other, there are two crumple zones.

All of the crumpling is dissipated energy. What doesn't get crumpled, goes into the driver's face.

Clearly, the situation with two crumple zones is superior. There's a limit to how much energy can be absorbed by crumpling. With two crumple zones, that limit is 2x, with one, 1x.

Therefore, damage to the driver or drivers is much worse in the brick wall scenario.

For another analogy, picture a go-kart, with a half-meter block of styrofoam, hitting a brick wall. Now, picture the same go-kart, hitting at the same speed, but this time, there's a meter of styrofoam.

That's the one-crumple and two-crumple scenarios.