1. Are brahmin siamese twins, or "two-brains-one-mind?"

2.What kind of evolutionary advantage could there be so that the post-war population is 100% brahmin and 0% one-headed?

1. Don't think it matters but I'm guessing om siamese twins.

Mutation is not evolution...
But lets not get into genetics, I'm quite sick of it for the moment.

Wolf mutation is actually evolution 10% of the time.

1. I believe they're single minded or can communicate in a way that you don't hear them.

2. If one head is destroyed shouldn;t the other survive?

I think it is one being with two heads.

Siamese twins only occurs as a horrible abberation.

Brahmin are meant to look like that.

There are somtimes one head brahmin but they are thought of as mutations.

Radiation caused them to evolve and mutate and have two heads and 8 stomaches and be stronger.

I would say having two heads and being stronger helped them.

They don't just have 2 heads, they have 8 stomach compartments (rather than 4) and Brahmin bulls have 4 testicles.

The Brahmin "improvements" are outside your survival of the fittest... Brahmin aren't generally wild, they're herded by people and protected by them. People like steak, Mincemeat and leather, and they're about as close to cows as you could get.