But this is generally a matter of poor individual rationality, and various arcane group structures that are supposed to compensate; rather than an inherent fact about cognitive processes somehow scaling better when chopped up into distinct brains. If that were literally more efficient, evolution would have designed humans to have four chimpanzee heads that argued with each other.

Note to self: Yes, you're right that the notion that evolution had a smooth path open to four-headed hominids is ludicrous, and the benefit of a well-designed four-headed human can't be evaluated based on this. But two chimpanzee brain hemispheres arguing with each other wouldn't be nearly as ludicrous, so don't reject Eliezer's argument because of this point.