Being Frank, (I should change my name to Frank),
I don't think Christian theology is sophisticated enough to handle a Satan/God duality. It barely handles Heaven/Hell duality with any plausibility, because both concepts are absent from Hebrew-Judaism. If they had attached their religion to a Buddhist pizza base, it might have worked.
It starts off with a capricious, jealous polytheistic collection of Gods who have "chosen peoples". They demand child and animal sacrifice, and can send plagues and famine to places they don't like, and part Red Seas. This is scratched out, with little sophistication, and then we pretend it's just one God, who has 2 or 3 names, and appears to still demand child sacrifice. Then, the religion carries on as a promise to deliver Israel from captors, if people follow a bunch of laws. This doesn't work, because Israel always seems to be enslaved, so they conclude that they are not following the laws well enough, or maybe it was a personal religion.
Because of that failure, they vote for personal religion, rather than a state religion, with no real explanation of how this could occur in the scripture. The religion then splits into 2 major factions: Jews vs Christians. Both groups stitch on heaven and hell, but the Christians make their hell worse, and try to convince monotheists to worship an extra man-god: Jesus; and have to pretend that he's really the same God as before, but nicer and with a different name. Then they introduce Satan as a third God, who you are not supposed to worship. They are then dumbfounded to come up with any convincing theology of how a third god can exist, or what his function is, but state categorically, that you are not supposed to worship him, because he is either evil, or against the mono Jesus-Elohim-Yahwey God (who also appears even more evil).
The terrible mistake that Christians made, was basing their theology on a published "open source" pizza base, that won't fit anything. Worse still, you can see all the eraser scrubbings in the theories, as they change their minds about core issues.