They do coexist Steve - they are in the same universe. In the Book of Job satan stands before God and has a confrontation with Him; how can that happen in they don't coexist?-Light(Christ) and darkness(Satan) cannot possibly co-exist!
2 Corinthians 6:14King James Version (KJV)
14 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?
The scripture you quote says they don't have any communion or fellowship not that they don't coexist.
Also in the Revelation satan shows up in the local church of Pergamos to persecute the children of God:
Revelation 2:13 I know thy works, and where thou dwellest, even where Satan's seat is: and thou holdest fast my name, and hast not denied my faith, even in those days wherein Antipas was my faithful martyr, who was slain among you, where Satan dwelleth.
Coexist - Yes.
Communion and/or fellowship - No.
HankD