Can relative velocity be found? Can you find the relative velocity between two cars with constant velocity of U1 and U2=-U1. In other words is there any kind of experiment you can do to understand that you are moving also and not just seeing the other car with 2U1?
 A: 
"...to understand that you are moving also and not just seeing the other car with 2U1"

That you are moving with respect to the ground? Yes. You can look at the ground and see that it is moving relative to you, so you must be moving relative to it.
A: "Can relative velocity be found?"
Yes.  More on that in a moment.
"is there any kind of experiment you can do to understand that you are moving also"
Theoretically, no.  However, in actual practice, an accelerometer will give you a pretty good idea of whether or not you are "moving" in the sense that you're thinking.  Ie, starting with both cars at rest, if your accelerometer never budges off zero, then the other car is moving at 2U.  If you read some acceleration for a while, then you can use those readings to calculate your new velocity relative to your original (assumed to be "stationary"), and then compare that to the other car.
"and not just seeing the other car with 2U1"
So getting back to where I started with this, the relative velocity is 2U1.  Consider the following three scenarios:
your car velocity | their car velocity
0                 | 2U1
2U1               | 0
U1                | U2 = -U1

In all three scenarios, the relative velocity is 2U1.  It's simply the difference between the two velocities.
