For example, the good/bad gods both manifest themselves in all sectors. You don't end up with a Goddess of Love, and a God of the Hunt for instance, accompanied by the Goddess of the Household, and so on endlessly. Both the good god and the bad god exhibit all those traits, but differently, as opposites, and depending on which side you are on you're going to view the one as better (for you) than the other.
Then there's the situation where you have one god but he is manifest in a male/female duality. That's a strong thread in Hinduism.
There is also a school of thought that reduces religion to true monotheism, a single God of good and evil exists only where the God does not exist. It's similar to believing Hell isn't a nice place because of distance from God, therefore there is no hope or hope of salvation.