Beth, I agree that it seems strange. I don't know much about antitrust law and can't explain why such collusion is allowed in this case, but there's definitely precedent for it. GM and Ford did it a decade ago and then again about four years ago. Other auto companies, such as Audi, BMW and Toyota, use external suppliers.
Beth, yes, these two are competitors. Since CAFE applies to everyone, and since the time scale is short, it makes sense. They don't do it often.
What is interesting is that I was just talking about this type of thing with my younger son. My car is 10 years old and has a four speed. My wife just bought a car with a six speed. Both have the auto-stick feature. I was making him aware that the first automatic transmissions had two speeds. That is the genesis of PRNDL. L was for low. The new car has PRNDS. The S is for second. Quite a lot of progress. Of course, we are getting close to the continuously variable transmission. This is doable, but very expensive.
At first blush, it seems strange to me that rivals Ford and GM would team up on a development effort that has such a major impact on each of their futures. Yet I suppose, given the seriousness of the new fuel economy standards, automotive OEMs are better served pooling their collective brain power and development resources to come up with some core foundational technology solution to the challenge in a much shorter time frame. Then they can refine/extend/improve the technology to differentiate their individual product lines when and where it makes sense.