I believe in Positive Energy and Negative Energy. If you open yourself up to either, then either good things or bad things can happen.
Then, of course, sometimes schtuff just happens.
I believe personally, that the story of Lucifer/Satan is the ultimate story of "See what happens when you go against God?". And that the idea of the Devil was instituted as a way of fearing people into going the right way.
I'm a good person. I treat others they way they treat me, I'm kind and considerate. And I do it all because I want to, not because I'm afraid of going to hell. (Which I believe is another fear tactic. I don't believe in hell, or heaven. When you die your energy goes out, your soul goes out and, yes, I do believe in reincarnation.)
Anyway, when explaining to a child the correct way to behave, you cannot simply say " Because you should." and expect them to understand and comply. You teach them cause and effect, and I believe that the concept of Satan is the same.
It would probably also help me to believe in Satan if I believed in God, but even when I was a Pagan and worshipped the God and Goddess there was no Satan in the mix. I don't think there has to be.
EL
I just reread your first post and realized I didn't even come close to answering your question.... I'll try now.
In order for it to be Dualism, don't both entities have to be on the same side?
If not, Satan wouldn't count anyway because, much as he'd like to be, he is NOT God or his equal. He has powers only because God allows him to because God knows in the end, we will choose him regardless.