What Joe says is essentially true. And I expect Joe already knows what I am about to explain, so is everyone ready for a small physics lesson.Double the speed is four times the impact. Energy = 1/2 mass x velocity squared.
This is also relativistic, so you add the two speeds together (100mph and 50mph). A collision at 150mph would transfer 9 times as much energy as one at 50mph. They would come together at 220 feet per second. There is also a mass differential between the two vehicles which is significant.
None of this matters much. The only important thing is that two people are dead, and many more lives have been shattered. Tragedy.
If a car runs into a steel plated brick wall (an example of a perfectly immoveable object).
The energy at 100 mph is 4X of a 50 mph crash and 150 mph is 9X.
Now a car at 50 mph running into a parked car, the parked car gets moved and so the effective energy of the crash is reduced. two cars of equal mass running head-on into each other is in effect equivalent to one car running into an immoveable object. The energy of the collision per car is equal to the 50 mph into a brick wall. So one car doing 100 into another doing 50, is not the same as 150 into the brickwall, but it is still a lot worse than two cars doing 50 head-on. As you have one car with 4x the energy of the other, total energy is only 5X when you add the two together, and then if we assume each car absorbs 1/2 the energy, then it's only 2.5X the energy of a 50 mph into a brick wall for each car. This last assumption is not likely to be 100% correct, but it's close enough for this physics lesson. If we take the square root of 2.5, we get 1.58, and this times 50 = 79, so the end result is more like 79 mph into a brickwall per vehicle.
and as Joe said, this makes no difference to the dead victims or their loved ones, this was a tragic but preventable crash. I feel for the survivors and hope the guilty party gets his due justice.