OK.... This is really driving me nuts. So I called my high school physics teacher. He confirmed what I thought to be true. The car travelling twice as fast will do about 4 times as much damage to the slower car as it will get. The problem above is that I was applying the wrong formulae above.

The Real issue, according to him, deals with kinetic energy, not momentum. An object of mass m moving with a speed v has a kinetic energy of ½mv². So you get ½mv² and ½mv`² Assuming speeds of 50kph and 100 kph and a mass of 1 unit (since they're the same) you get KE=1250 vs KE=5000. These units are in joules per kilogram of the mass. So if it's say 1500 kg trucks you get Car 1 issuing 1,875,000 Joules of energy into the other object. Car 2 transfers 7,500,000 Joules of energy into the other car. Since both cars come to a complete stop, the kinetic energy must be transfered into the other vehicle as damage, not movement. If you'd like, I'll scan and post the pages from my physics book.