I believe there is a little confusion here....
When two car
at the same speed and the same weight have head-on collision, the collision speed (or Velocity) is the summation of two speeds and the
TOTAL damage is like one car is hitting the wall with the total speed (V1+V2), but since there are two cars involved the damage will be split between two cars so it's like each car hit the wall with the same speed. If cars have different speeds and different weights then damage (on each car) is based on the other car's momentum ( p= m* V).
watch this:
and this: