A satanist is by definition not an atheist.
If you take satanist to mean somebody who worships Satan and Atheist to mean a Naturalist who thinks nothing Supernatural exists, yeah sure. But people sometimes use the words more broadly, so I said "most" to just avoid getting into a pedantic discussion about the distinction...but it seems I can't resist...
There may be some who call themselves both an Atheist and Satanist:
1) Somebody who thinks God exists but just hates God and worships Satan and considers themselves an Atheist because to them the term means Anti-Theist or against God.
2) Somebody who does not believe in anything supernatural but who makes out that they are a "Satanist" to mess with people or to act out against God (yes people sometimes do even while holding the intellectual position that He doesn't exist) or get attention.
But I think it safe to say most who call themselves Atheists do not fall into one of these categories, since very few people are Satanists.