Does 2 Corinthians 6 say you cannot have fellowship with God and demons?
Yes. The context presents 2 Corinthians 6 as teaching that fellowship with God and demons cannot coexist. It stresses necessary separation and declares that one cannot have fellowship or communion with demons, connecting this to the Lord's table and the table of demons.
Les explains that in I Corinthians 10, the Gentiles' sacrifices are offered to demons, not to God, because they know nothing of God. Paul therefore says he would not have believers have fellowship with demons. Les then points ahead to II Corinthians chapter 6, where he says the same thing: you cannot have fellowship with God and demons; there has to be separation. He adds that you cannot have fellowship or communion with demons, and he is building his case for the communion table. Verse 21 reinforces it: you cannot drink the cup of the Lord and the cup of demons; you cannot be partakers of the Lord's table and the table of demons. It just cannot be done.
Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?
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