Lets do the math. Take 2 cars that each are relatively safe and get 30 mpg each such as the Chevy Impala. The pair of them driving 1000 miles would consume 1000/30 x 2 gallons of gas or 66 gallons of gas. Now lets take a pair of vehicles that together average 30 mpg. I’ll use a hypothetical 12 mpg vehicle (think Hummer) to average 30 mpg we’ll pair it with a 48 mpg Honda Civic (48 +12 = 60 / 2 = 30). Over the course of 1000 miles the Hummer will consume just over 83 gallons of gas. So there’s no need to even calculate the Civic’s mileage. Even if it ran on sunshine nothing it could do could offset the Hummer’s terrible mileage. By itself it consumed more than a pair of Impala’s so the idea that somehow we can offset to achieve a goal of reducing gas consumption doesn’t even come close to passing the “back of napkin” test.
Another real problem with CAFE standards is that in order to get these offsets car manufacturers need to get us to buy ultra efficient cars. This is how we got the Geo Metro a few years back. So instead of having vehicles of similar sizes on the road we have lots of cars at the extremes. This leads to a partially correct view that small cars are dangerous. They don’t have to be, but when built with ultra high MPG as the main goal, they are too small to stand up to a high speed collision with a large or medium size vehicle. This perception of small cars as dangerous leads more people to buy large vehicles “to be safe”. This increases the danger to those occupants of the smallest cars and increases the danger to the occupants of the large SUV who otherwise might be in something more sensible such as an Impala. The Impala is safer than the Hummer because it doesn’t roll over as easily AND people in Impalas are more likely to wear their safety belts.
CAFE standards that allow averaging simple MPG numbers rather than averaging gallons of gas consumed lead to unintended consequences that we can’t live with.

Actually, VW is moving forward with a limited run of a vehicle based on this concept.

http://gas2.org/2011/01/31/vw-will-build-100-formula-xl-1-261-mpg-vehicles/