From:codellw@mindspring.com

i've always thought that if a human attained godhood, a human would become

a demon to balance >it out ala the "Doublet System". Perhaps the demon
Oshima or the demoness Sayoko?

Some spoiler here, so don't read below if you don't want to.

The 'Doublet System' is selective. Only individuals from both sides who are
willing to be bound by it are used, it is voluntary ( as far as I can tell).
Second, they use children, probably because their innocence allows them to
bond better. Third, the Doublet binding is a ceremony that takes place in a
room with only the 2 people involved. After they leave, all memories of the
event are either erased or sealed (can't be sure which, either way they're
not supposed to remember).

Pretty useless IMO, since if a diety or demon is killed, there's less than 1
in a thousand chance that someone on the other side will die as well.