One of the most interesting episodes I saw was when they were testing something Jamie said in an earlier episode: That if two trucks collide at 55 MPH, it's like one truck hitting a brick wall at 110 MPH. At first I thought "duh, everyone knows that's true" and I continued to think that as they set up experiments, right until they were about to let two clay blocks swing into each other at which point a light bulb lit up above my head, and so I quickly hit the pause button and thought about what was going to happen, and realized that since each block of clay was simply going to stop the movement of the other, each was going to end up in the same condition it would have been in had it simply slammed into the "immovable object" instead, and thus two vehicles each going 55 MPH in a head-on collision is exactly like just one vehicle hitting a brick wall in a 55 MPH collision. ...and I suppose it's solvable with math too, given e = m * v, and so if two objects slowing down one unit of speed yields two units of energy, or one unit per object, then one object slowing down two units of speed yields four units of energy, which is four times as much, even though the difference in speeds is identical in each case. ...but I was certainly misinformed about how it worked, and I don't think I was the only one, so it was totally worth doing an episode on, indeed it was one of my favorites since I actually learned something.
First, E = 1/2 m*v^2, not m*v, although your later statement seems to acknowledge that.
Second: you are correct that the two situations are not the same, because the energy in the center-of-motion (zero momentum) frame of the two vehicles is what matters (you can think about this as the kinetic energy that is available to deform the vehicles in the crash, leaving them with a lot of bent metal and no momentum after the crash.)
With two trucks moving toward each other at equal and opposite velocities, the zero momentum frame is the just the ground, where the total energy is m*v^2 (twice the energy of each individual vehicle).
In the case of hitting a wall, the wall has effectively infinite mass, so the zero momentum frame is moving with an infinitesimal velocity toward the truck, and the total energy is 2*m*v^2 (where "v" is still 55 MPH and the multiplier come from squaring the factor of two in front of it to get the full 110 MPH of the single truck).
So in the case of hitting a brick wall, there is twice the energy available. This is quite different, conceptually, from the explanation you've given, which is wrong. In the case of a vehicle hitting a brick wall at v = 55 MPH the energy is just 1/2 m*v^2, not m*v^2 as in the case of two colliding vehicles, or 2*m*v^2 as in the case of a vehicle at 2v hitting the wall.
The history of science teaches us that what is intuitive to any particular person is unrelated to the best way of understanding the world, and your reasoning is a nice example of this: it got you part way toward a correct conclusion, but fell short of the full understanding that the general principles of Newtonian physics give us.