—would indeed be more complex and might benefit by virtue of 2 brains, 2 personalities. 4 ears, 4 arms, etc.—
But would it have been better? I understand that half of all two brained entities end up in divorce.
You mean two brained humans, lots of animals either pair for life or never pair.
If we use a married couple as the example you can add procreation and reduced expenses living together to the benefits. And following the divorce, one organism will probably end up better than before.